GitHub profile now-playing widget: enrich the now-playing cloud sync

[LargeModGames]

What

Powers the new public GitHub profile widget (https://spotatui.com/widget/<username>.svg, opt-in via the spotatui.com dashboard) by enriching the client's now-playing cloud sync.

• The push to POST /api/sync/now-playing now includes album, image_url, duration_ms, progress_ms with a client sample timestamp (progress_at_ms), is_playing, and is_live. The backend (already deployed) extrapolates the widget's progress bar from the timestamp between heartbeats and stays compatible with older clients.
• New PlaybackSnapshot.is_live flag, set only by internet radio, so the widget can render a LIVE badge instead of a progress bar.
• Push triggers extracted into a unit-tested NowPlayingTracker: track change (as before), play/pause flips (immediate paused card), seek discontinuities > 5s, and a heartbeat that now also runs while paused so the widget shows "paused" instead of decaying to offline. Note: this keeps the dashboard online dot lit while paused with the app open.
• Stopping playback mid-session now clears the cloud status right away instead of waiting for app exit.
• The collector uses the shared timeout-guarded HTTP client instead of an un-timed reqwest::Client::new().
• SPOTATUI_SYNC_BASE_URL env override points all sync endpoints at a local backend for development.

Testing

• cargo fmt / clippy -D warnings / cargo test under the CI feature set: 361 tests pass, including new tests for the payload wire format and every tracker trigger.
• cargo check on default and all-sources feature sets (the radio is_live branch).
• Wire contract exercised end-to-end against the real backend locally (curl with identical JSON through the deployed migration and widget renderer).