Add push notifications for scheduled expenses

[maraf]

Summary

Implements Web Push notifications to alert users when scheduled expense templates are due today. Based on the notification pattern from neptuo/Recollections#395.

API Changes

• NotificationsController — REST endpoints for notification settings and push subscriptions
  • GET /api/notifications — Get current settings
  • PUT /api/notifications — Toggle global notifications
  • PUT /api/notifications/expense-templates — Configure expense template notifications (enable, preferred hour, timezone)
  • POST /api/notifications/subscriptions — Register browser push subscription
  • DELETE /api/notifications/subscriptions — Revoke subscription
• PushNotificationSender — WebPush/VAPID-based push delivery with auto-revocation of expired endpoints
• ExpenseTemplateNotificationBackgroundService — Background service that ticks every 15min, checks due templates per user's timezone/preferred hour, deduplicates via dispatch table
• EF Entities — PushSubscription, UserNotificationSettings, UserNotificationExpenseTemplateSettings, ExpenseTemplateNotificationDispatch (with unique index for dedup)

Blazor Client Changes

• Service workers — Push event handlers for showing notifications + click-to-navigate
• JS interop — Money.Notifications module (subscribe, unsubscribe, permission check, timezone detection)
• PushNotificationInterop — C# wrapper for JS Push API calls
• Notifications page (/user/notifications) — Settings UI with global toggle, expense template toggle, preferred hour selector, browser subscription management
• Navigation — Added to user dropdown menu

Architecture

Maintains separation between:

• Accounts layer — Owns subscriptions, settings, and push delivery
• Money domain — Owns "due today" logic (recurrence calculation via ReadModelContext)

Setup

To enable push notifications, generate VAPID keys and configure in appsettings.json:

npx web-push generate-vapid-keys

Then set Notifications:PublicKey and Notifications:PrivateKey.

TODO (follow-up)

• EF migration (needs to be generated in environment with DB access)
• Badge icon asset for Android

Closes #617
Closes #672
Closes #673