fix(webview): relay the request Content-Length so length-only dashboards get the body

The proxy hands undici the request body as a stream and dropped the browser's
content-length, assuming undici recomputes it. For a stream it cannot: undici
frames it as Transfer-Encoding: chunked. A dashboard that reads only
Content-Length (Python's http.server, many small Go/C servers) then saw an
empty body on every POST through a web tab. Found on a WoL dashboard whose
Shut down posted {"confirm":true} and was refused for a missing flag on every
click through Codeman, while curl straight at it worked.

buildUpstreamRequestHeaders now puts the browser's content-length back when a
body goes upstream (relayBodyLength), it is a plain number, and no
transfer-encoding contradicts it. GET/HEAD never carry it. undici 6.28 frames
the stream with that length and fails cleanly on a mismatch. Side effect:
a DELETE/OPTIONS with content-length 0 now goes upstream with neither header
nor body instead of an empty chunked body.

test/webview-proxy-body-framing.test.ts pins it end to end against a
Content-Length-only upstream; it fails on the old code with
"expected 'chunked' to be null".

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -541,9 +541,9 @@ Further detail on the Docker Compose supervisor (`docker-compose`), see also `do
 1. **Root-absolute URLs built at runtime.** `<base href>` only governs URLs the HTML parser resolves; `fetch('/api/data')` bypasses it and lands on Codeman's root. That is how most dashboards talk to their own backend. The `Referer`-keyed 404 fallback is only a rescue (it fires after every real route missed, and only when the browser sends a usable `Referer`), so the fix is `runtimeUrlShim()`: a small script injected right after `<base>` that rebases root-absolute and same-origin-absolute URLs into the prefix. It removes the whole class inside the iframe instead of trading security for it. ⚠️ It must be injected even when the page ships its OWN `<base>` (an early return there silently breaks exactly the pages that need it most). ⚠️ **The DOM sinks are as load-bearing as `fetch`.** Patching only `fetch`/`XHR`/`WebSocket`/`EventSource` leaves `container.innerHTML = '<img src="/api/hero?slug=x">'` and `img.src = '/api/slide'` untouched, and neither of the other layers can reach those either (`<base>` never applies to root-absolute URLs, and `rewriteHtml()` only ever sees the INITIAL document, never markup built later by page script). The symptom is precise and easy to misdiagnose as an upstream fault: the dashboard's **data** loads while every **image** stays broken. So the shim also wraps `innerHTML`/`outerHTML`/`insertAdjacentHTML`, `setAttribute`/`setAttributeNS`, and the `src`/`srcset`/`href`/`poster`/`data`/`action` property setters, with a `MutationObserver` as a last net for sinks not patched above. Every rewrite routes through the same idempotent `rw()`, which matters because unlike the server-side rewrite this one sees markup that may ALREADY be proxied (a page re-injecting its own `outerHTML` would otherwise double-prefix). The DOM half is pinned in jsdom by `test/webview-proxy.test.ts`; `curl` cannot see any of it.
 2. **CORS on same-host requests.** An opaque-origin document treats EVERY request as cross-origin, including to the very host it was served from, so its `fetch`/XHR are CORS-checked and its preflights carry `Origin: null`. Static subresources (script/css/img) are NOT CORS-checked, which is why the page renders while its API calls die with an opaque `net::ERR_FAILED`. `buildProxyCorsHeaders()` echoes the origin (omitting `allow-credentials` for `null`, which browsers reject in combination), upstream `access-control-*` headers are dropped (they describe the dashboard's origin, not the frame's), and the proxy answers preflights itself rather than relaying them. ⚠️ `registerSecurityHeaders` answers EVERY `OPTIONS` with a bare 204 before routing, and its CORS block only emits headers for localhost origins, so that short-circuit **must** exempt a valid webview capability or every preflight fails. `curl` cannot reproduce any of this because curl does not enforce CORS.
 
-**Rewrites, each load-bearing** (pure + unit-tested in `src/web/webview-proxy.ts`): drop `x-frame-options` and the CSP `frame-ancestors` directive (the point of the proxy); drop `content-encoding`/`content-length` because undici's `fetch` already decoded the body (forwarding them makes the browser gunzip plaintext); rewrite `Location` for same-origin redirects only, handing CROSS-origin redirects back unchanged so this never becomes an open relay; rebase `Set-Cookie` `Path` onto the prefix and drop `Domain`; inject `<base href>` and rebase root-absolute `src`/`href`/`action`. `resolveUpstreamUrl()` returns null on anything escaping the upstream origin.
+**Rewrites, each load-bearing** (pure + unit-tested in `src/web/webview-proxy.ts`): drop `x-frame-options` and the CSP `frame-ancestors` directive (the point of the proxy); drop the response's `content-encoding`/`content-length` because undici's `fetch` already decoded the body (forwarding them makes the browser gunzip plaintext); relay the REQUEST's `content-length` whenever a body goes upstream (`relayBodyLength`, no `transfer-encoding` present), see Fastify specifics; rewrite `Location` for same-origin redirects only, handing CROSS-origin redirects back unchanged so this never becomes an open relay; rebase `Set-Cookie` `Path` onto the prefix and drop `Domain`; inject `<base href>` and rebase root-absolute `src`/`href`/`action`. `resolveUpstreamUrl()` returns null on anything escaping the upstream origin.
 
-**Fastify specifics.** The proxy lives in an **encapsulated plugin scope** with `removeAllContentTypeParsers()` + a `'*'` pass-through parser, so raw bodies relay byte-for-byte while the root instance keeps its JSON parsing (and keeps `text/plain` RAW, which was a real CSRF hole once). One `app.route({ method:'GET', handler, wsHandler })` serves both HTTP and the WebSocket upgrade; **HEAD must NOT be declared** on the sibling route because `exposeHeadRoutes` already derives it and the duplicate is a startup error. ⚠️ **Every exit path in the proxy handler RETURNS `reply.send(...)`**: the handler is `async`, and a bare `return` after `reply.send(stream)` resolves the handler promise to `undefined` before the stream is consumed, so Fastify answers with an EMPTY body. HTML survives that (synchronous string payload) while every streamed asset comes back zero-length, which is a genuinely confusing failure. The root-absolute fallback hangs off `setNotFoundHandler`, so real Codeman routes always win.
+**Fastify specifics.** The proxy lives in an **encapsulated plugin scope** with `removeAllContentTypeParsers()` + a `'*'` pass-through parser, so raw bodies relay byte-for-byte while the root instance keeps its JSON parsing (and keeps `text/plain` RAW, which was a real CSRF hole once). ⚠️ **Bytes, and the FRAMING too**: undici sends a stream body with no known length as `Transfer-Encoding: chunked`, and a server that reads only `Content-Length` (Python's `http.server`, many small Go/C ones) then sees an EMPTY body. That broke every POST to such a dashboard until 2026-10-21 (a WoL page's Shut down lost its `{"confirm":true}` and was refused on every click, while curl straight at it worked), so `buildUpstreamRequestHeaders` puts the browser's `content-length` back, pinned end to end by `test/webview-proxy-body-framing.test.ts` against a length-only upstream. One `app.route({ method:'GET', handler, wsHandler })` serves both HTTP and the WebSocket upgrade; **HEAD must NOT be declared** on the sibling route because `exposeHeadRoutes` already derives it and the duplicate is a startup error. ⚠️ **Every exit path in the proxy handler RETURNS `reply.send(...)`**: the handler is `async`, and a bare `return` after `reply.send(stream)` resolves the handler promise to `undefined` before the stream is consumed, so Fastify answers with an EMPTY body. HTML survives that (synchronous string payload) while every streamed asset comes back zero-length, which is a genuinely confusing failure. The root-absolute fallback hangs off `setNotFoundHandler`, so real Codeman routes always win.
 
 **Frontend** (`src/web/public/webview-tabs.js`, load order 12.5): web tabs render into `#sessionTabs` carrying `data-webview-id` instead of `data-id`, so every session-tab path (drag-and-drop, alerts, badges) skips them. ⚠️ `_renderSessionTabsImmediate()`'s `canIncremental` check must include the web-tab comparison: the session-only comparison is vacuously "unchanged" whenever session count is stable, most visibly at ZERO sessions (`0 === 0`), where opening a dashboard would never draw its tab. ⚠️ `isActive` for a session tab is `id === activeSessionId && !activeWebviewId`: `activeSessionId` stays set while a web tab is showing (the terminal keeps streaming underneath), so without that clause the debounced re-render lights two tabs at once. Frames stay MOUNTED while hidden (LRU-evicted past `maxLiveFrames`) so tab switching never reloads a dashboard.