Toasts: non-blocking notifications replace MessageBoxes + log-only failures (TASK 24)

TASKS.md

@@ -1058,6 +1058,44 @@ The tasks below are ordered by dependency and risk. Each task builds on the prev
 
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+## TASK 24 — Toast notifications (in-app, non-blocking)
+
+### Status: ✅ Done (shipped 2026-08-23, branch `task24-notifications`)
+
+Replace blocking MessageBoxes with in-window toasts; promote actionable log-only failures.
+
+### Requirements:
+1. Library: **Notification.Wpf 11.0.0** (Platonenkov fork of Federerer/Notifications.Wpf) — MIT, active, targets net8.0-windows. Notice #10 added to `THIRD-PARTY-NOTICES.txt`.
+2. Placement: bottom-right above the footer (`NotificationArea x:Name="ToastArea"`, last child of MainWindow's root grid → topmost z-order, `Grid.RowSpan=4`, `Margin="0,0,12,96"`, outside the preview Viewbox), max 4 stacked.
+3. Behavior: Info ~4s / Success ~4s / Warning ~8s auto-dismiss; **Error sticky until dismissed**.
+4. Styling: dark card tints matching the health-banner language (slate `#3a3b52` info, green `#1d5c38` success, amber `#b8860b` warning, dark-red `#8f1f1f` error); corner radius 8; keep-visible-on-hover.
+5. Threading: all Show calls marshal via a Dispatcher captured at construction (encoder/pump events fire off-thread).
+
+### Migrations (MessageBox → toast):
+- Webcam acquire failure ×2 (`AddWebcamToStagedSceneAsync`, `SwapWebcamIdentityAsync`) → Warning
+- Image read failure (`PickImageBytes`) → Warning
+- Sign-in unsuccessful → Warning; sign-in failed → Error
+
+### Promotions (log-only → toast, each keeps its AppLog line):
+- Go-live prep ×3: reusable stream unavailable / broadcast insert failed / prep exception → Error
+- Frame-pump death while live (`OnFramePumpFailed`) → Error
+- Mic missing at startup (`StartMicCaptureAsync`) → Warning (once)
+- Premium lapse at startup re-validation → Warning
+- Saved-session refresh failure (signed out notice) → Info
+
+Deliberately left alone: offline license re-validation skip (log-only — would spam every launch); SocialsDialog slot-delete YesNo confirm (stays modal).
+
+### Tests:
+- Unit: pure severity→request mapping (`NotificationServiceTests`) — area routing, lifetimes, tints.
+- Integration (the ONE): real window-hosted `ToastArea` + real service — Info auto-dismisses, Error sticks (`NotificationAreaIntegrationTests`, RealApp collection).
+
+### Library facts (cost a hunt — see also ai.md):
+- Area routing matches the area's XAML `Name` against the request's `AreaName`; unknown name silently drops.
+- `NeverExpires()` = `ExpirationTime = TimeSpan.MaxValue` (not null); `NotificationColor.ToHex()` returns `#AARRGGBB`.
+- Overlay-window shutdown caveat avoided entirely by using the in-window area mode.
+
+---
+
 ## Backlog (future versions)
 
 1. v1.1 — Stream Deck / Loupedeck integration (requires hotkey foundation from TASK 20)