feat(settings): rebuild App Settings as a rail over one scrolling document

The modal had grown to 8 tabs that wrapped onto two rows on desktop and
became a horizontal scroller on phones, with a "Display" mega-tab holding
11 sections and ~35 controls. Local Echo sat 60% down it, and the model
settings were split across two tabs whose three controls fought each
other (the 1M Opus toggle's own hint said it was "ignored when a Claude
Model is selected above").

Replaced with a left rail that is a TABLE OF CONTENTS over one scrolling
document: every section stays mounted, the rail follows the scroll, and
find-in-page works across the whole thing. Nine sections:

  Terminal & Input (Local Echo is the first row of the first section)
  Appearance, Header & Panels, Models, Agents & CLIs,
  Notifications, Voice, Shortcuts, System

Models are now one page. The picker is a card grid of BASE models with a
single "1M context window" switch; context becomes a property of the
chosen model and composes back into `claudeModel` as `base + [1m]`, which
retires the precedence trap. Thinking effort is a segmented control on
the same page, and the old Models tab (task routing) becomes a collapsed
Advanced block under it.

The 12 header-button toggles and the 8 panel toggles become chip grids,
which is most of the old Display tab reclaimed. Rows now say whether a
setting is per-device or synced, stated once per group.

Phones drop the rail for a sticky jump pill that names the current
section and opens a jump list, move Save into the header (the bottom
action bar cost 60px), and render groups as one inset rounded list with
hairline dividers instead of a stack of bordered cards.

Load and save are untouched: every control keeps its id, so
openAppSettings()/saveAppSettings() work as before. Model cards and the
effort segment are views over hidden <select>s that stay the source of
truth. test/app-settings-structure.test.ts pins that contract, plus the
rail hooks admin-ui.js injects the multi-user Users section into.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -260,6 +260,8 @@ Frontend JS modules have `@fileoverview` with `@dependency`/`@loadorder` tags. L
 
 **Per-device vs synced settings**: the `displayKeys` set in settings-ui.js is a **client-side merge policy**, not a wire filter. A display key seeds from the server only when localStorage has no value for it, which is what prevents one device overwriting another; `showPlanUsageLimits` is additionally `delete`d from the incoming payload outright. Separately, `SettingsUpdateSchema` is `.strict()` and simply **does not declare** `skin`, `showFileViewerButton`, `showCronButton`, `webglRendererEnabled`, `localEchoEnabled`, `cjkInputEnabled`, or `extendedKeyboardBar`, so sending one of those is a validation error. The rest (`showResponseViewer`, `showPlanUsageLimits`, `language`, and most `show*` keys) ARE in the schema and do persist server-side; they are per-device by client policy only. ⚠️ Adding a new per-device setting means deciding **both** questions: membership in `displayKeys`, and presence in the schema.
 
+**App Settings modal** (`#appSettingsModal`): a left rail that is a **table of contents over ONE scrolling document**, not a tab switcher. Every section stays mounted (`.set-section`, ids `settings-terminal|appearance|layout|models|clis|notifications|voice|shortcuts|system`), the rail follows the scroll, and `switchSettingsTab(id)` keeps its historical name but SCROLLS instead of hiding. Phones swap the rail for the sticky `#appSettingsJump` pill (compact layout at ≤860px in mobile.css). ⚠️ **The load/save contract is `getElementById` by id**: `openAppSettings()`/`saveAppSettings()` read every control by a fixed id, so moving a control between sections is free but renaming or dropping one silently stops it loading or saving. `test/app-settings-structure.test.ts` is the static guard. ⚠️ Model cards (`#appSettingsModelCards`) and the effort segment are **views over hidden `<select>`s** that remain the source of truth; the cards hold the BASE model and the "1M context window" switch composes `base + [1m]` back into `claudeModel`, which is what retires the old "takes precedence over the toggle below" trap. ⚠️ `.modal-tabs`/`.modal-tab-btn`/`.modal-tab-content` are still used by `#sessionOptionsModal` and `#createCaseModal`; the settings rail uses its own `set-*` classes and must not restyle them. ⚠️ `admin-ui.js` injects the multi-user Users entry into `.set-rail-items` + `.set-doc`, so those hooks must survive any restructure.
+
 **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`) 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 + 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)
 
 **Gesture control** (camera hand-tracking overlay, opt-in, default OFF): `CODEMAN_GESTURE=1` makes the feature *available*; `gestureControlEnabled` turns it on. The bundle is injected by `renderIndexHtml` only when enabled, which is why that method is `async` and reads settings with `readSettings(true)` (a fresh read: a post-save reload lands inside the 2s cache TTL and would otherwise render the pre-toggle state). **Source lives in `packages/gesture-control/`; edit there, run `npm run build:gesture`, and commit the regenerated bundle** because dev serves the committed bundle with no runtime bundler. The MediaPipe wasm + model are fetched separately and gitignored. ⚠️ Keep `MP_VERSION` in `fetch-gesture-assets.mjs` in sync with `@mediapipe/tasks-vision`. → [architecture-invariants#gesture-control-the-source-package](docs/architecture-invariants.md#gesture-control-the-source-package)