COD-121 unified session list: persistent Session Manager modal (slice A, unit 3)

Adds a header-reachable Session Manager so the complete session list is
available mid-session, not only on the welcome screen.

- index.html: always-on header button (.btn-session-manager) + #sessionManagerModal
  (mirrors the Away Digest modal) with a search box + results list.
- panels-ui.js: openSessionManager()/closeSessionManager()/_loadSessionManagerList()
  — loads GET /api/sessions/unified (limit 200), renders via the unit-2
  _buildHistoryItem (rich items, mode/LIVE badges, open->select / closed->resume),
  debounced search wired to the endpoint's q= param, empty/error states. A
  modal-scoped Escape listener closes it even when focus is in the search input;
  backdrop click and item click also close it.
- app.js: closeSessionManager() added to the global Escape chain.
- styles.css: modal + list styling (items reuse .history-item).

Verified on an isolated beta instance (Playwright): the header button opens the
modal, it lists 200 sessions from /api/sessions/unified, a no-match query issues
?q= to the server and yields 0 items, clearing restores the list, clicking an
item closes the modal and routes resume/select, and Escape closes it. Gates:
tsc 0, lint 0, frontend-syntax + public-asset format clean, 17 tests pass.

src/web/public/app.js

@@ -827,6 +827,7 @@ class CodemanApp {
         this.closeAllPanels();
         this.closeHelp();
         if (this.attachmentHistoryDrawerOpen) this.closeAttachmentHistory();
+        this.closeSessionManager();
       }
 
       // Option/Alt session navigation uses physical key CODES, not e.key, so macOS