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.

ViewModels/MainViewModel.Streaming.cs

@@ -175,7 +175,7 @@ public partial class MainViewModel : ViewModelBase
     /// and the old gate needed a sign-in or a lit pill — signed-out creators found a
     /// blank bar). No dead ends: an unarmed Start records (local needs no account);
     /// sign-in lives in the Start context menu.</summary>
-    public bool ShowPrimaryStartButton => IsOffline && !IsRecording;
+    public bool ShowPrimaryStartButton => IsOffline && !IsRecording && !IsOffAirAfterDrop;
 
     /// <summary>Either output selected makes the button actionable.</summary>
     public bool CanStartSession => RecordPillOn || OnAirPillOn;