docs: document the settings surface and repoint the moved settings paths

A docs pass landed in this worktree while the preview was up (a respawn loop on
the throwaway session it was serving), and it is the documentation this work
needed, so it is reviewed and kept rather than thrown away.

- docs/architecture-invariants.md gains a "Settings surface" section: the one
  `:is()` scope and why the id-only list preserves specificity, the anatomy,
  the two meanings of the rail, the deliberate two sizes, the phone strip, the
  Add Case adapter, the flex-summary chevron trap, the Respawn ordering, the
  retired tab chrome, and the live preview's clone-the-chip-icon rule.
- Settings paths are repointed everywhere they moved: Display -> Header &
  Panels (header buttons, cron, multi-monitor, response viewer, file viewer),
  Settings -> App Settings -> System -> Updates, Panels -> Header & Panels ->
  Cross-session features (Read My Mind), Display -> Terminal & Input (gesture
  control), Claude Model -> Models -> New Claude sessions.
- Stale counts refreshed (route modules, frontend modules, type files, config
  files) and the typecheck script named.
- browser-testing-guide gains the three modal ids and the `set-*` selectors.
- The styles.css block comment covers all three modals.

Two claims it got wrong are corrected here: an external-CLI session opens
Session Options on the Session tab (`switchOptionsTab('context')`), not
Summary - measured in the browser - and the Cron toggle lives under Header &
Panels -> Scheduling, with no "Header Displays" step under it any more.

docs/browser-testing-guide.md

@@ -300,6 +300,11 @@ For reference when writing browser tests:
 .xterm                   // Terminal container
 #helpModal               // Help modal
 #appSettingsModal        // Settings modal
+#sessionOptionsModal     // Session Options (same set-* surface)
+#createCaseModal         // Add Case (same set-* surface)
+.set-rail-item           // Rail entry: scrolls in App Settings, switches in the other two
+.set-section             // A settings section (`.hidden` on the inactive ones outside App Settings)
+.set-row                 // One setting: label + description left, control right
 .modal-content           // Modal content
 .modal-close             // Modal close button
 .header-brand .logo      // Logo text