a11y: app toast notifications become a live region for screen readers

showAppToast spawns / reuses a single floating toast at the bottom-
right of the window for transient status (e.g. "1 new VOD auto-queued",
"Cannot start recording", etc). The toast had no a11y semantics —
screen readers never announced it, so the entire transient-feedback
channel was silent for AT users.

Promoted the toast container to a live region:
- role="status" for info toasts + aria-live="polite" so the reader
  waits for a natural break in current speech before announcing
- role="alert" for warn toasts + aria-live="assertive" so the reader
  interrupts whatever it was saying (matches the visual amber-left-
  border meaning — warn IS urgent)
- aria-atomic="true" so the reader announces the whole message at
  once instead of attempting to diff against the previous toast

Critical detail: aria-live attributes have to be in place BEFORE the
text changes for AT to register the change as a live-region update.
The current implementation now sets role / aria-live first and only
then writes the new textContent.

WCAG 4.1.3 — Status Messages.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/renderer.ts

@@ -689,14 +689,28 @@ function showAppToast(message: string, type: 'info' | 'warn' = 'info'): void {
         toast = document.createElement('div');
         toast.id = 'appToast';
         toast.className = 'app-toast';
+        // Live region — screen readers announce the toast text whenever
+        // it changes. Warn toasts go through aria-live="assertive" so the
+        // reader interrupts whatever it was speaking; info toasts use
+        // "polite" so they wait for a natural break in current speech.
+        toast.setAttribute('role', 'status');
+        toast.setAttribute('aria-live', 'polite');
+        toast.setAttribute('aria-atomic', 'true');
         document.body.appendChild(toast);
     }
 
-    toast.textContent = message;
     toast.classList.remove('warn', 'show');
     if (type === 'warn') {
         toast.classList.add('warn');
+        toast.setAttribute('role', 'alert');
+        toast.setAttribute('aria-live', 'assertive');
+    } else {
+        toast.setAttribute('role', 'status');
+        toast.setAttribute('aria-live', 'polite');
     }
+    // Setting textContent AFTER the aria-live attribute is in place
+    // ensures the change is captured as a live-region update by AT.
+    toast.textContent = message;
 
     requestAnimationFrame(() => {
         toast?.classList.add('show');