feat: session sharing over the relay (M12) — Share tab

Add a Share tab that hosts or joins a read-only live session through the
rigdoctor-relay over WebSocket (QtWebSockets), gated by the Gitea access token.

- gui/share_page.py: Start shared session (host: get a code, stream snapshot +
  health + inventory) and Enter share code (guest: view a host's data read-only)
- core/share.py: host_full_frame / host_snapshot_frame + guest_html renderer
- config: relay_url (default wss://rigdoctor.jesseyvanofferen.com)
- setup: token now powers updates AND sharing — hint asks for read:user +
  read:repository scopes (relay validates the account via Gitea)
- main_window: Share nav tab + socket cleanup on close
- tests for the relay frame builders and guest HTML

Verified end-to-end against the deployed relay (host code -> guest frame).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/MODULES.md

@@ -18,7 +18,7 @@ Status: ⬜ not started · 🟦 designing · 🟨 in progress · ✅ done
 | M10 | Desktop GUI | Desktop UI | **python3-pyside6** | all | P2 | 🟨 |
 | M11 | Tray / menu-bar applet | Desktop UI | **python3-pyside6** (+ AppIndicator on GNOME) | all | P2 | ⬜ |
 | M9 | Installer | (meta) | none | all | P1 | 🟨 |
-| M12 | Session sharing / remote assist | Sharing | none (Tier 3: tmate/sshx) | all | P3 | ⬜ |
+| M12 | Session sharing / remote assist | Sharing | none (Tier 3: tmate/sshx) | all | P3 | 🟨 |
 | M13 | Auto-update | (core) | none (stdlib; user-local file swap) | all | P3 | 🟨 |
 | ~~M7~~ | ~~Stress / repro~~ | — | — | — | — | ❌ dropped (D7) |