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>

CHANGELOG.md

@@ -5,6 +5,22 @@ All notable changes to RigDoctor are recorded here. Format follows
 (`MAJOR.MINOR.PATCH`, pre-1.0). `__version__` and `pyproject.toml` must match the git
 release tag (so the auto-updater, D18, can compare versions).
 
+## [0.6.0] - 2026-05-21
+### Added
+- **Session sharing over the relay (M12)**: a **Share** tab — *Start shared session* (host)
+  hands you a short code and streams a read-only live view; *Enter share code* (guest) joins
+  someone else's session and views their sensors/health/inventory. Both connect outbound over
+  WebSocket to the relay (`relay_url`, default `wss://rigdoctor.jesseyvanofferen.com`), gated
+  by your Gitea access token — no port forwarding. Read-only.
+
+## [0.5.0] - 2026-05-21
+### Added
+- **Session sharing (M12, Tier 2)**: `rigdoctor share serve` starts a **read-only** live view
+  (sensors auto-refresh + health report + inventory) over a local HTTP server, gated by a
+  random share token. Bind to localhost for local testing, or to all interfaces behind a
+  user-chosen tunnel (Tailscale/cloudflared/SSH) for remote help. No actions, no terminal.
+  (Tier 1 export and Tier 3 gated terminal still to come — D16.)
+
 ## [0.4.1] - 2026-05-21
 ### Fixed
 - Checkbox contrast: a checked checkbox is now a clear accent-filled box with a checkmark