feat(desktop): macOS menu bar tray with live session state and mini-chat UX

Add an always-visible macOS status bar (tray) item that surfaces OpenChamber's
live state and acts as a quick launcher, plus a series of related desktop UX
fixes around mini-chat, window routing, notifications and shortcuts.

Tray (new):
- Monochrome template cube glyph that adapts to the menu bar light/dark.
- Icon-driven activity indicator: a smooth, eased, infinite "breathing" fill
  while sessions are busy; a static filled cube when finished sessions are left
  unread; a plain outline when idle. Text counters next to the icon only for
  actionable states (pending approvals, errors).
- Menu lists active sessions (status glyph, branch, unread count) with overflow
  rolled into a submenu; pending permission/question approvals with inline
  Allow once / Allow always / Deny; quick actions (New Session, New Mini Chat,
  Show OpenChamber, Quit). Header shows the active instance name
  ("Local OpenChamber" or the remote host label) for multi-window clarity.
- Session list sourced from the global (cross-project) sessions store, sorted by
  last-updated, independent of which directories are currently open; live
  status/unread/branch merged in from directory sync stores where available.
  Sub-session (multi-run) activity rolls up to the parent row.
- Event-driven updates (global store + directory stores + notifications +
  registry) with a short debounce; polling kept only as a slow safety net.

Tray/window routing:
- Opening a session from the tray targets the surface the user was last on: if a
  mini-chat is active it switches that existing window to the session in place
  (no new window); otherwise the main window (revealed without a reload).
- app.activate (dock click) restores the last-focused/minimized window instead
  of spawning a new main window; only creates one when nothing is left.
- "Open in main window" and tray session-open now create the main window when
  none exists, queuing the session as a pending deep-link so it opens once the
  fresh renderer is ready.

Mini chat:
- New Mini Chat is now a customizable shortcut, exposed in Settings > Shortcuts,
  in the File menu (hint only, renderer owns the binding), and in the tray.
- Themed splash backdrop on window open to remove the white flash / flicker;
  dismissed once content is ready, leaving the content's single cube logo.
- Mini-chat can switch sessions in place via openchamber:open-session.

Notifications:
- The active/selected session only counts as "seen" when the window is focused,
  so turns completing while the app is backgrounded raise an unread marker;
  refocusing the window clears it.

packages/electron/resources/icons/tray/tray-glyph.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32" fill="none"
+     stroke="#000" stroke-linejoin="round" stroke-linecap="round">
+  <g stroke-width="2.3">
+    <path d="M16 2.5 L28.5 9.5 L28.5 23 L16 30 L3.5 23 L3.5 9.5 Z"/>
+    <path d="M3.5 9.5 L16 16.25 L28.5 9.5"/>
+    <path d="M16 16.25 L16 30"/>
+  </g>
+  <!-- thinner stroke for the diamond on the top face -->
+  <path stroke-width="1.5" d="M16 6.5 L21.5 9.4 L16 12.3 L10.5 9.4 Z"/>
+</svg>