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/styles.css

@@ -5443,6 +5443,34 @@ kbd {
   padding: 1rem;
 }
 
+/* Session Manager modal (COD-121) */
+.session-manager-modal {
+  max-width: 720px;
+  width: min(92vw, 720px);
+  max-height: 70vh;
+  display: flex;
+  flex-direction: column;
+}
+
+.session-manager-modal .modal-body {
+  display: flex;
+  flex-direction: column;
+  gap: 0.6rem;
+  overflow: hidden;
+  padding: 1rem;
+}
+
+#sessionManagerSearch {
+  flex: 0 0 auto;
+}
+
+.session-manager-list {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+  overflow-y: auto;
+}
+
 .away-digest-ranges {
   display: flex;
   flex-wrap: wrap;