feat(gnome): GNOME Shell extension for shared /menubar tray panel

[yesme]

Summary

Adds a GNOME Shell extension that reuses the existing shared /menubar Web UI (same surface as macOS) instead of building a separate Linux tray.

• Top bar: compact multi-provider % with monochrome provider icons
• Hover / click: opens the shared menubar panel via a thin WebKitGTK host (panel_host.py)
• Theming: white symbolic SVGs + ColorizeEffect tinted from panel foreground so icons follow light/dark panel colors
• System theme in WebView: pass ?system_prefers_dark=1|0 from panel appearance (WebKitGTK often reports light incorrectly under GNOME)
• Close (X): WebKit onwatchAction bridge hides the popover; no longer falls back to navigating to / (which loaded dashboard/login inside the tray)
• Wayland reloads: mtime cache-bust import so indicator.js changes apply via disable/enable without logout (after extension.js is loaded once)

Install (local)

./gnome-extension/install.sh
# Wayland: log out once if extension.js itself changed; then disable/enable is enough for indicator-only updates

Requires the onwatch daemon on http://127.0.0.1:9211.

Test plan

• Enable extension; top bar shows provider icons + % matching tray-title API
• Dark panel: icons/text are light gray-white (not pure black)
• Light panel: icons/text remain dark
• Hover/click opens /menubar popover
• With menubar theme System, dark panel opens dark menubar (not forced light)
• Click X only closes the popover (does not open /login or replace content with dashboard)
• Open Dashboard from menu opens the full UI in the system browser
• ./app.sh --test or at least menubar page/bootstrap tests under internal/web