feat(panes): desktop host — real windows and the system tray

Registers a `desktop` pane host at priority 20, above the browser pop-up
host (10) and the dock (0), whenever the Electron bridge exposes
feedBackDesktop.panes. A popped-out pane then gets a real BrowserWindow:
it remembers where you put it, can float above everything, minimizes to
the system tray, and appears in the tray's menu.

In a plain browser — or on an older desktop build that predates the
bridge — this file registers nothing and the browser pop-up host handles
detach exactly as before. Nothing else in the pane system changes. That
is what the host registry is for.

Two things only this realm can decide, so it owns them:

- The user closed a pane window (or it crashed). Close the pane, or the
  dialog its pop-out chip hid never comes back and the user is left with
  no way to reach their own UI.
- The tray asked to toggle a pane it has no window for. Main cannot know
  what opening one means — the pane might belong in the dock — so it asks.

Unlike a browser pop-up, this host needs no user gesture, so it sets
autoRestore: true — a pane you left popped out comes back popped out,
where you left it, on the next launch.

Pairs with got-feedback/feedBack-desktop#103.

Signed-off-by: topkoa <topkoa@gmail.com>

static/v3/index.html

@@ -1327,6 +1327,10 @@
     <script defer src="/static/panes/pane-manager.js"></script>
     <script defer src="/static/panes/pane-dock.js"></script>
     <script defer src="/static/panes/pane-window-host.js"></script>
+    <!-- Desktop host: registers above the browser pop-up host when the Electron
+         bridge is present (real BrowserWindow + system tray), and registers
+         nothing at all in a browser. -->
+    <script defer src="/static/panes/pane-desktop-host.js"></script>
     <script defer src="/static/panes/pane-hub.js"></script>
     <script defer src="/static/panes/pane-chip.js"></script>
     <script defer src="/static/panes/pane-launcher.js"></script>