feat(notifications): add overlay notifications with position config

- Add Catppuccin Macchiato overlay notification stack with 3s transient timeout
- Add `notifications.overlayPosition` config (top-left | top | top-right)
- Route startup tokenization and subtitle annotation status through configured surfaces
- Deduplicate rapid subtitle mode toggle notifications
- Change `both` to mean overlay + system; add `osd-system` as legacy alias for old behavior
- Keep `osd`/`osd-system` as config-file-only legacy values; Settings UI offers overlay/system/both/none

src/types/notification.ts

@@ -0,0 +1,53 @@
+export const SETTINGS_NOTIFICATION_TYPE_VALUES = ['overlay', 'system', 'both', 'none'] as const;
+
+export const NOTIFICATION_TYPE_VALUES = [
+  ...SETTINGS_NOTIFICATION_TYPE_VALUES,
+  'osd',
+  'osd-system',
+] as const;
+
+export const OVERLAY_NOTIFICATION_POSITION_VALUES = ['top-left', 'top', 'top-right'] as const;
+
+export type SettingsNotificationType = (typeof SETTINGS_NOTIFICATION_TYPE_VALUES)[number];
+export type NotificationType = (typeof NOTIFICATION_TYPE_VALUES)[number];
+export type OverlayNotificationPosition = (typeof OVERLAY_NOTIFICATION_POSITION_VALUES)[number];
+
+export type OverlayNotificationVariant = 'info' | 'success' | 'warning' | 'error' | 'progress';
+
+export interface OverlayNotificationAction {
+  id: string;
+  label: string;
+}
+
+export interface OverlayNotificationPayload {
+  id?: string;
+  title: string;
+  body?: string;
+  variant?: OverlayNotificationVariant;
+  position?: OverlayNotificationPosition;
+  persistent?: boolean;
+  timeoutMs?: number;
+  actions?: OverlayNotificationAction[];
+}
+
+export interface OverlayNotificationDismissPayload {
+  id: string;
+  dismiss: true;
+}
+
+export type OverlayNotificationEventPayload =
+  | OverlayNotificationPayload
+  | OverlayNotificationDismissPayload;
+
+export function isNotificationType(value: unknown): value is NotificationType {
+  return typeof value === 'string' && NOTIFICATION_TYPE_VALUES.includes(value as NotificationType);
+}
+
+export function isOverlayNotificationPosition(
+  value: unknown,
+): value is OverlayNotificationPosition {
+  return (
+    typeof value === 'string' &&
+    OVERLAY_NOTIFICATION_POSITION_VALUES.includes(value as OverlayNotificationPosition)
+  );
+}