feat: unified guest mode with AuthGate + createGuestMode composable

Major refactor of ManaCore's (app) layout for guest mode support:

- New createGuestMode() composable in shared-stores — encapsulates
  welcome modal state, nudge timer, and notifications in one call
- Replace monolith onMount with AuthGate + handleAuthReady callback:
  Phase A (auth-independent): DB init, migration, uload, dashboard
  Phase B (auth-dependent): sync, user settings, onboarding
  Phase C (guest-only): welcome modal + registration nudge
- Root route / always redirects to /home (no auth check)
- PillNav shows login button for guests, user email for auth users
- GuestWelcomeModal with manacore-specific features
- SessionWarning only renders for authenticated users
- Proper cleanup via onDestroy

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-auth-ui/src/components/GuestWelcomeModal.svelte

@@ -40,6 +40,7 @@
 
 	/** Default features per app (German) */
 	const defaultFeaturesDE: Record<string, string[]> = {
+		manacore: ['Alle deine Apps an einem Ort', 'Quelloffen & unabhängig', 'Privat by Design'],
 		contacts: ['Alle Kontakte an einem Ort', 'Quelloffen & unabhängig', 'Privat by Design'],
 		chat: ['Dein persönlicher KI-Assistent', 'Quelloffen & unabhängig', 'Privat by Design'],
 		todo: ['Organisiere deinen Alltag', 'Quelloffen & unabhängig', 'Privat by Design'],
@@ -72,6 +73,7 @@
 
 	/** Default features per app (English) */
 	const defaultFeaturesEN: Record<string, string[]> = {
+		manacore: ['All your apps in one place', 'Open-source & independent', 'Private by design'],
 		contacts: ['All your contacts in one place', 'Open-source & independent', 'Private by design'],
 		chat: ['Your personal AI assistant', 'Open-source & independent', 'Private by design'],
 		todo: ['Organize your day', 'Open-source & independent', 'Private by design'],