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/index.html

@@ -118,7 +118,7 @@
         </div>
         <button class="btn-icon-header btn-redraw-terminal btn-redraw-terminal--hidden" onclick="app.restoreTerminalSize()" title="Redraw terminal to fit current screen (Ctrl+Shift+R)" aria-label="Redraw terminal"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><polyline points="1 4 1 10 7 10"/><polyline points="23 20 23 14 17 14"/><path d="M20.49 9A9 9 0 0 0 5.64 5.64L1 10m22 4l-4.64 4.36A9 9 0 0 1 3.51 15"/></svg></button>
         <button class="btn-icon-header btn-response-viewer-header btn-response-viewer-header--hidden" onclick="app.toggleResponseViewer()" title="View last response" aria-label="View last response"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z"/><circle cx="12" cy="12" r="3"/></svg></button>
-        <button class="btn-icon-header btn-away-digest" onclick="app.openAwayDigest()" title="Away Digest" aria-label="Open away digest"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M8 6h13"/><path d="M8 12h13"/><path d="M8 18h13"/><path d="M3 6h.01"/><path d="M3 12h.01"/><path d="M3 18h.01"/></svg></button>
+        <button class="btn-icon-header btn-away-digest" onclick="app.openAwayDigest()" title="Away Digest" aria-label="Open away digest"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M8 6h13"/><path d="M8 12h13"/><path d="M8 18h13"/><path d="M3 6h.01"/><path d="M3 12h.01"/><path d="M3 18h.01"/></svg></button> <button class="btn-icon-header btn-session-manager" onclick="app.openSessionManager()" title="Session Manager" aria-label="Open session manager"><svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><polyline points="12 2 2 7 12 12 22 7 12 2"/><polyline points="2 17 12 22 22 17"/><polyline points="2 12 12 17 22 12"/></svg></button>
         <button class="btn-icon-header btn-attachments-history btn-attachments-history--hidden" id="attachmentsHistoryBtn" onclick="app.toggleAttachmentHistory()" title="Attachments" aria-label="Open attachment history" aria-expanded="false">
           <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"/></svg>
           <span class="attachment-history-badge" id="attachmentHistoryBadge" style="display:none;">0</span>
@@ -1956,6 +1956,21 @@
       </div>
     </div>
 
+    <!-- Session Manager Modal -->
+    <div class="modal" id="sessionManagerModal">
+      <div class="modal-backdrop" onclick="app.closeSessionManager()"></div>
+      <div class="modal-content session-manager-modal">
+        <div class="modal-header">
+          <h3>Sessions</h3>
+          <button class="modal-close" onclick="app.closeSessionManager()" aria-label="Close session manager">&times;</button>
+        </div>
+        <div class="modal-body">
+          <input type="search" id="sessionManagerSearch" class="search-input" placeholder="Search sessions by name, prompt, or path&hellip;" autocomplete="off" maxlength="200">
+          <div id="sessionManagerList" class="session-manager-list"></div>
+        </div>
+      </div>
+    </div>
+
     <!-- Token Stats Modal -->
     <div class="modal" id="tokenStatsModal">
       <div class="modal-backdrop" onclick="app.closeTokenStats()"></div>