feat: App Navigator sidebar for quick app switching (#1026) - #2055

Summary

Closes #1026. Adds a hamburger-triggered "App Navigator" sidebar — a searchable, category-grouped
list of apps — so users can switch between apps without leaving an active chat, replacing the
DOM-injected custom-scripts.html hack from the aurahub fork with a native React component.

• client/src/shared/components/AppNavigator.jsx — the sliding panel (300px, 85vw max on
  mobile), backdrop, search box, and category groups. Opens via a new header button
  (AppNavigatorToggleButton.jsx) or Ctrl/Cmd+B; closes on Escape, backdrop click, route
  change, or selecting an app.
• client/src/shared/contexts/AppNavigatorContext.jsx — open/closed state (never persisted,
  per the issue's FR-12) and per-category collapsed state (persisted to localStorage, mirroring
  the existing admin SidebarContext.jsx pattern).
• client/src/hooks/useAppNavigator.js — wraps the context and registers the Ctrl/Cmd+B
  shortcut, ignored while typing in a text field so it doesn't fight "bold text" shortcuts.
• client/src/shared/utils/appNavigatorGroups.js (+ appNavigatorGroups.test.js) — pure
  search-filter and category-grouping logic, covered by a plain-node unit test wired into
  npm run test:quick as test:app-navigator. Categories reuse appsList.categories.list as the
  single source of id/name/color — the new appNavigator.categoryOrder config only reorders them,
  it doesn't redefine category metadata.
• Keyboard navigation: Up/Down between app items via the existing useKeyboardNavigation hook
  (no new keyboard-handling code needed).
• Admin UI: new UI Customization > App Navigator tab
  (AppNavigatorCustomization.jsx) to enable/disable the sidebar and reorder categories.
• Config: new migration V078__add_app_navigator_config.js seeds
  ui.json's appNavigator: { enabled: true, categoryOrder: [] } for existing installs;
  server/defaults/config/ui.json ships it for fresh installs. Documented in docs/ui.md and
  docs/releases/5.5.0/features.md.
• i18n: added an appNavigator section to shared/i18n/en.json/de.json.

Per the implementation-plan comment already on the issue, this PR intentionally scopes to the
original PRD (app switcher + categories + search) and does not attempt the larger "Phase 1C"
three-column ChatGPT-style shell proposed in a later comment (tracked separately as #1491) —
building that in the same PR would conflate two different-sized efforts.

Note on branch history

This branch (claude/charming-fermat-js2fkf) already had three unrelated, unmerged commits from
earlier sessions with no PR ever opened for them (migration version-collision fix for #1685, an
audit-log gap fix, and a dead-code removal in LoginPage). Per repo convention I kept rather than
discarded that prior work, so this diff includes those in addition to the App Navigator feature.
Happy to split them into separate PRs if preferred.

Test plan

• New client/src/shared/utils/appNavigatorGroups.test.js (11 assertions) — search
  filtering, category ordering/fallback, and the "Other" bucket — via
  npm run test:app-navigator (now part of test:quick).
• server/tests/migrations-runner.test.js — 46/46 passing, confirms the new V078 migration
  doesn't collide with the version-uniqueness check added earlier on this branch.
• npx eslint on all changed/new files — 0 errors (a few pre-existing-style warnings only,
  confirmed present before this change too).
• npx prettier --check — clean.
• npx vite build (client) — succeeds with no errors.
• Booted the dev server: migration V078 applies cleanly, ui.json gets
  appNavigator: { enabled: true, categoryOrder: [] }.
• Manual browser verification of the sidebar UI (open/close, search, category collapse,
  keyboard nav) was not performed in this sandboxed session — no way to drive a real browser
  here.

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