feat(stream): one-click Back on air — a new broadcast, honestly labelled (TASK 32·3)

The last resilience slice. When a drop ends the broadcast, YouTube cannot resume it, so the only way
back is a new one — and that is what the app now offers, in one click.

- `Services/BroadcastTitle.cs` — the resume marker is applied to the title the show was using, and any
  existing marker is REPLACED rather than stacked ("Show (resumed)" → "Show (resumed 2)"), with the base
  truncated so the whole title fits YouTube's 100-character limit. Otherwise a channel grows two
  identically named videos and the creator has to explain it.
- `MainViewModel.BackOnAirCommand` — reuses the ordinary go-live path with the saved details and the
  cached stream: new broadcast, new VOD, no dialog. It appears only after a drop ended the session and
  the ordinary start button steps aside, so there is exactly one way forward.
- The running line goes red OFF AIR (not amber) in that state; the button reads "● Back on air".

Tests: `BackOnAirTests` — first re-air marks the title, a second drop counts instead of stacking, the
100-character limit holds, an empty title still works, and the post-drop state exposes exactly one
start affordance. FAST gate: 552 passed, 0 failed, 0 warnings.

TASK 32 is now complete: fragmented MP4, blip retry, poll-driven RECONNECTING, ingest pre-flight, Back
on air.

TASKS/task-32-stream-resilience.md

@@ -39,9 +39,14 @@ the API — creator ruling 2026-09-01): no lying in the copy.
    Test: a fake status provider drives the sign through live → reconnecting → off air; a `chatEndedEvent`
    triggers an immediate status check rather than a decision of its own.
 
-3. ☐ **One-click Back on air** — the relabeled button fires the existing go-live path with saved
+3. ✅ **One-click Back on air** (shipped 2026-11-05) — the relabeled button fires the existing go-live path with saved
    `Broadcast.*` + cached stream, dialog skipped: new broadcast, new VOD (accepted cost, stated
    in-product). Test: seeded saved form + fake stream service → click creates + pushes, no dialog.
+   **Shipped:** `Services/BroadcastTitle.cs` (pure — resume marker replaces rather than stacks, base
+   truncated to YouTube's 100 characters) and `MainViewModel.BackOnAirCommand`, which reuses the ordinary
+   go-live path with the saved details and the cached stream: new broadcast, new VOD, no dialog. The
+   button appears only after a drop ended the session, in place of the normal start button, and the
+   ordinary start button steps aside so there is exactly one way forward. Tests: `BackOnAirTests`.
 4. ✅ **Crash-safe recording** (shipped 2026-11-05) — fragmented MP4 on the record block (`-movflags
    frag_keyframe+empty_moov+default_base_moof`): plain MP4's trailing moov atom means a power loss
    kills the WHOLE file. Rename-on-stop unchanged. Closes the TASK 18 "MKV as an option" design-note