ci(visual): mark linux-gnome experimental until Mutter blue-paint is fixed (#36)

## Summary
Mark the Linux (GNOME) visual job as `experimental: true` (matching
Cinnamon's convention) so its failures no longer block the workflow.

GNOME Mutter paints the fixture window as a solid Adwaita-blue rectangle
regardless of every fix tried so far:
- ✅ `transparent: false`
- ✅ `backgroundColor: '#FFFFFF'`
- ✅ `frame: false`
- ✅ `useContentSize: true`
- ✅ `mb.window.focus()` after positioning
- ✅ HTML body `background: rgb(255, 255, 255)`

The test still passes (white pixels exist elsewhere on screen via the
`globalWhite` fallback) but the saved screenshot doesn't show the
white-box-with-black-square design the way the other 13 platforms do.
Likely an Electron+Mutter rendering conflict that needs deeper
investigation; not worth gating CI on while we figure it out.

Author: afonsojramos

.github/workflows/visual-tray.yml

@@ -162,11 +162,21 @@ jobs:
                       build_snixembed: true
                       panel_warmup: 12
                       post_ready_delay: 6000
+                    # GNOME is experimental: Mutter paints the fixture window
+                    # as an opaque Adwaita-blue rectangle regardless of our
+                    # backgroundColor / HTML body color / frame:false / focus()
+                    # combination. The test still passes via the globalWhite
+                    # fallback (white pixels exist elsewhere on screen) but the
+                    # saved screenshot doesn't show the white-box-with-black-
+                    # square design the way the other 13 platforms do. Marking
+                    # experimental so future regressions don't block merges
+                    # until someone figures out the Electron+Mutter conflict.
                     - de: gnome
                       label: Linux (GNOME)
                       key: linux-gnome
                       apt: gnome-shell gnome-shell-extension-appindicator
                       panel_cmd: bash -c 'gnome-extensions enable ubuntu-appindicators@ubuntu.com 2>/dev/null || true; gnome-shell --x11'
+                      experimental: true
                     # Budgie: noble ships budgie-desktop 10.9, whose default panel layout
                     # already includes the System Tray applet. The applet is a pure client
                     # of org.freedesktop.StatusNotifierWatcher — and in 10.9+ that watcher