feat(tauri): proactive update notifications via toast (P5-40)

TauriUpdateFeature component in ClientNonUIFeatures checks for updates
on mount and every 12h (skips if checked within the window). On update
available, fires a Lotus toast: "Lotus Chat vX.Y.Z is ready to install."
Clicking the toast calls install(). No-op on web (isTauri guard).

Also adds optional onClick to ToastNotif type and wires it in
LotusToastContainer so custom click handlers can skip hash navigation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/state/toast.ts

@@ -8,6 +8,7 @@ export type ToastNotif = {
   roomName: string;
   roomId: string;
   hashPath?: string; // overrides window.location.hash navigation when set
+  onClick?: () => void; // custom click handler; skips hash navigation when set
 };
 
 const baseAtom = atom<ToastNotif[]>([]);