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>

test/admin-ui.test.ts

@@ -28,7 +28,7 @@ async function bootWith(me: Record<string, unknown>) {
   const dom = new JSDOM(
     `<!doctype html><body>
       <button id="adminPanelBtn" class="btn-admin-panel btn-admin-panel--hidden"></button>
-      <div class="modal" id="appSettingsModal"><div class="modal-tabs"></div><div class="modal-body"></div></div>
+      <div class="modal" id="appSettingsModal"><nav class="set-rail"><div class="set-rail-items"></div></nav><div class="set-doc" id="appSettingsDoc"></div></div>
     </body>`,
     { url: 'http://localhost/', runScripts: 'outside-only' }
   );
@@ -45,22 +45,23 @@ async function bootWith(me: Record<string, unknown>) {
 }
 
 describe('admin-ui boot', () => {
-  it('exposes the identity and injects the Users tab for a multi-user admin', async () => {
+  it('exposes the identity and injects the Users section for a multi-user admin', async () => {
     const { win } = await bootWith({ username: 'root', role: 'admin', multiUser: true, mustChangePassword: false });
     expect(win.__codemanUser).toMatchObject({ username: 'root', role: 'admin', multiUser: true });
-    const btn = win.document.querySelector('[data-tab="settings-users"]');
+    // The settings modal is a rail over one document: a rail entry, not a tab.
+    const btn = win.document.querySelector('[data-section="settings-users"]');
     expect(btn).toBeTruthy();
     expect(win.document.getElementById('settings-users')).toBeTruthy();
   });
 
-  it('does NOT inject the Users tab for a regular user', async () => {
+  it('does NOT inject the Users section for a regular user', async () => {
     const { win } = await bootWith({ username: 'joe', role: 'user', multiUser: true, mustChangePassword: false });
-    expect(win.document.querySelector('[data-tab="settings-users"]')).toBeFalsy();
+    expect(win.document.querySelector('[data-section="settings-users"]')).toBeFalsy();
   });
 
   it('does NOT inject the Users tab in single-user mode', async () => {
     const { win } = await bootWith({ username: 'admin', role: 'admin', multiUser: false, mustChangePassword: false });
-    expect(win.document.querySelector('[data-tab="settings-users"]')).toBeFalsy();
+    expect(win.document.querySelector('[data-section="settings-users"]')).toBeFalsy();
   });
 
   it('shows the change-password modal when mustChangePassword is set', async () => {