test(mobile): the gate checks the File Viewer button's phone default again (#809)

Until #809 a display:none rule in mobile.css and its KNOWN_PHONE_HIDDEN
entry kept the File Viewer button off phones, and the gate checked both.
With that rule gone, the handheld default in getDefaultSettings() and the
html.mobile-init startup rule are the whole lock, and only the browser
test saw them, which npm test does not run.

The policy test now loads the real getDefaultSettings() on a handheld and
on a desktop, checks that every reader of the setting goes through the
device default, and that mobile.css keeps the startup hide.

Docs: architecture-invariants, src/web/CLAUDE.md and the wiki say the
button is off on a phone until it is switched on there, and that a window
narrower than 600px on a device that is not a handheld now shows it.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

src/web/CLAUDE.md

@@ -60,7 +60,7 @@ Frontend JS modules have `@fileoverview` with `@dependency`/`@loadorder` tags. L
 
 **Settings surface** (`#appSettingsModal` + `#sessionOptionsModal` + `#createCaseModal`): one `set-*` language shared through a single `:is(...)` id scope in styles.css. App Settings' rail is a table of contents over ONE scrolling document (`switchSettingsTab` scrolls); Session Options and Add Case really switch (`switchOptionsTab` / `switchCaseModalTab`), and their larger per-modal size blocks are the design, not drift. ⚠️ **The load/save contract is `getElementById` by id**: renaming or dropping a control id silently stops it loading or saving. ⚠️ The Session Options "Session" entry still keys off `context` (label-only rename). ⚠️ Add Case keeps its legacy `.form-row` markup via an adapter; every `<details>` there needs `.set-adv-chev` plus both marker suppressions. ⚠️ Model cards and the effort segment are views over hidden `<select>`s, which stay the source of truth. ⚠️ `.modal-tabs*` classes are retired; `admin-ui.js` needs `.set-rail-items` + `.set-doc` to survive any restructure. Guard: `test/app-settings-structure.test.ts`. → [architecture-invariants#settings-surface-app-settings-session-options-add-case](../../docs/architecture-invariants.md#settings-surface-app-settings-session-options-add-case)
 
-**Header button visibility**: most header controls are opt-in and hidden by a marker class (`btn-multimonitor--hidden`, `btn-response-viewer-header--hidden`, `btn-file-viewer--hidden`, `btn-cron--hidden`, and the desktop-gated `btn-split--hidden` / `btn-tile-grid--hidden`, which also need a `@media (max-width: 1179px)` backstop) that `applyHeaderVisibilitySettings()` (settings-ui.js) toggles after settings load; the multi-monitor button is instead stripped at render by `renderIndexHtml`. ⚠️ Hiding must go through the marker class: the base rules are `display:inline-flex !important`, so an inline style cannot override them. Current desktop default is WS/CPU/MEM + File Viewer + Tiles (1180px and wider) + gear, with the token chip and lifecycle-log button OFF. ⚠️ New header controls must not leak onto phones; `test/mobile-header-buttons-policy.test.ts` is the static guard. → [architecture-invariants#header-button-visibility-multi-monitor-response-viewer-file-viewer-cron](../../docs/architecture-invariants.md#header-button-visibility-multi-monitor-response-viewer-file-viewer-cron)
+**Header button visibility**: most header controls are opt-in and hidden by a marker class (`btn-multimonitor--hidden`, `btn-response-viewer-header--hidden`, `btn-file-viewer--hidden`, `btn-cron--hidden`, and the desktop-gated `btn-split--hidden` / `btn-tile-grid--hidden`, which also need a `@media (max-width: 1179px)` backstop) that `applyHeaderVisibilitySettings()` (settings-ui.js) toggles after settings load; the multi-monitor button is instead stripped at render by `renderIndexHtml`. ⚠️ Hiding must go through the marker class: the base rules are `display:inline-flex !important`, so an inline style cannot override them. Current desktop default is WS/CPU/MEM + File Viewer + Tiles (1180px and wider) + gear, with the token chip and lifecycle-log button OFF. ⚠️ New header controls must not leak onto phones; `test/mobile-header-buttons-policy.test.ts` is the static guard. The one opt-in exception is File Viewer (#759): `mobile.css` no longer hides it at phone width, so it stays off a phone only through the handheld default in `getDefaultSettings()` and the `html.mobile-init` startup rule (both pinned by that test), and shows once the setting is switched on on that phone. → [architecture-invariants#header-button-visibility-multi-monitor-response-viewer-file-viewer-cron](../../docs/architecture-invariants.md#header-button-visibility-multi-monitor-response-viewer-file-viewer-cron)
 
 **Terminal font weight** (`terminalFontWeight` / `terminalFontWeightBold`, per-device, default = xterm's own `normal`/`bold`): Claude Code's markdown bold is a bare `ESC[1m`, so the weight step is its only cue. `CodemanTerminalFont.resolveWeights()` (constants.js, pure) resolves each slot against **its own** xterm default. ⚠️ The `@font-face` for `fonts/jetbrains-mono-variable.woff2` must stay declared `100 800` (the browser synthesizes from the descriptor, not the file); narrowing it silently makes the setting a no-op. ⚠️ A live save must reach both echo overlays (`refreshFont()`) and open Agent Teams panes. ⚠️ Leave `_awaitTerminalFont()` untouched. Test: `test/terminal-font-weight.test.ts`. → [architecture-invariants#terminal-font-weight](../../docs/architecture-invariants.md#terminal-font-weight)
 

src/web/public/styles.css

@@ -14427,8 +14427,8 @@ html:is([data-skin="paper-gray"], [data-skin="solarized-light"], [data-skin="cat
   display: none !important;
 }
 
-/* "File Viewer" header button — opt-in (App Settings → Header Displays), hidden
-   by default. Clicking it toggles the file browser panel. Same marker pattern as
+/* "File Viewer" header button: on by default on a desktop, opt-in on a phone
+   (App Settings → Header & Panels). Clicking it toggles the file browser panel. Same marker pattern as
    the response viewer: a base inline-flex !important so an inline style can't
    override it, and a more-specific marker rule to hide. */
 .btn-file-viewer {