fix: apply the review items for #678 at merge

docs: document the SSE connect timeout next to the staleness watchdog in src/web/CLAUDE.md and docs/architecture-invariants.md (clear order, replaced-EventSource guards, offline handler leaves the timer)
test: rename the offline case to "connectSSE() while offline cancels the pending timer" so it names the path it drives

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

src/web/CLAUDE.md

@@ -86,7 +86,7 @@ Frontend JS modules have `@fileoverview` with `@dependency`/`@loadorder` tags. L
 
 **Connection-loss UI** (`computeConnectionLossUi()` in constants.js, writer `_updateConnectionLossUi()` in app.js): the service worker serves the cached app shell, so an unreachable server (phone off the tailnet, VPN down, server stopped) used to render a normal-looking empty dashboard whose only tell was the 8px header dot, which reads as "no sessions", not "no connection". Two surfaces now: a full-screen **overlay** while no server state has loaded this page load (nothing behind it is worth preserving), and a non-blocking **banner** once it has (the terminal scrollback stays readable). ⚠️ A **2.5s grace** is load-bearing: a COM deploy restarts the server and SSE is back in ~200ms, and a banner on every deploy trains the user to ignore it. `navigator.onLine === false` skips the grace, since that is never a blip. Retry re-arms SSE **and** the terminal WS (`planWsReconnect` can 'give-up', and the SSE backoff caps at 30s).
 
-**SSE staleness watchdog** (`computeSseStale()` in constants.js, `_checkSseStale()` + a 5s interval in app.js): an `EventSource` can stop delivering without erroring, so the client forces a reconnect when nothing arrives. ⚠️ The server keepalive must stay the named `sse:heartbeat` event (`cleanupDeadClients()`, sse-stream-manager.ts), never an SSE comment, which `EventSource` cannot observe; its no-op client listener must stay registered. ⚠️ Judge staleness only while `connected` and online (the loop breaker). ⚠️ The liveness stamp lives inside `addListener`. ⚠️ Clear the interval only at the top of `connectSSE()`, or intervals stack. → [architecture-invariants#sse-staleness-watchdog](../../docs/architecture-invariants.md#sse-staleness-watchdog)
+**SSE staleness watchdog** (`computeSseStale()` in constants.js, `_checkSseStale()` + a 5s interval in app.js): an `EventSource` can stop delivering without erroring, so the client forces a reconnect when nothing arrives. ⚠️ The server keepalive must stay the named `sse:heartbeat` event (`cleanupDeadClients()`, sse-stream-manager.ts), never an SSE comment, which `EventSource` cannot observe; its no-op client listener must stay registered. ⚠️ Judge staleness only while `connected` and online (the loop breaker). ⚠️ The liveness stamp lives inside `addListener`. ⚠️ Clear the interval only at the top of `connectSSE()`, or intervals stack. ⚠️ The watchdog cannot see an attempt that never OPENS (no response headers: no `open`, no `error`, never `connected`), so each attempt is bounded by `SSE_CONNECT_TIMEOUT_MS` (15s, `CodemanSseStale.CONNECT_TIMEOUT_MS`) and then retried through `_handleSseConnectionFailure()`, the same backoff `onerror` uses. Its timer is cleared at the top of `connectSSE()` BEFORE the offline early return, on `open`, and in `_handleSseConnectionFailure()`; the timer and the handler both ignore an `EventSource` that has been replaced. The window `offline` handler leaves it running on purpose (`navigator.onLine` can report offline on a working network). Tests: `test/sse-connect-timeout.test.ts`. → [architecture-invariants#sse-staleness-watchdog](../../docs/architecture-invariants.md#sse-staleness-watchdog)
 
 **Z-index layers** (keep new overlays consistent with this stack): local echo overlay (7; with local echo on it also draws the iOS IME composition preview, as an underlined tail after its pending text via `setComposition`), iOS IME composition preview span when local echo is off (6 inside `.xterm-helpers`, whose own z-index 5 is its EFFECTIVE layer, so it sits UNDER the overlay and must never be used while the overlay shows text), terminal touch-selection bar (900, below floating agent windows), subagent windows + split picker menu + Tiles count menu (1000), plan agents (1100), mobile/tablet fixed header (1200), modals on ≤768px (1300, must beat the fixed header), log viewers (2000), connection-loss overlay (2500), image popups (3000), response viewer (5000, backdrop 4999), folder grid (5090, above the response viewer it opens from, below the file preview its tiles open), file-preview overlay (5100, must outrank the response viewer that launches it), toasts/path picker (10000+), custom-model center-status banner (10001; its `[hidden]` must re-assert `display: none` or `dismiss()` leaves an invisible click-blocker), custom-model swap-confirm/context-warning modals (10010). → [architecture-invariants#z-index-layers](../../docs/architecture-invariants.md#z-index-layers)