feat(push): fix notification delivery and add detailed messages with auto-subscribe

[hechi]

Summary

• Fix push notifications not being delivered due to Base64 encoding mismatch (standard vs URL-safe) and CORS-blocked X-Device-Id header
  (now passed as query param)
  • Replace generic "List updated" notifications with action-specific messages: item added (✚), checked (✓), unchecked (↩), edited (✎),
    deleted (✕), and completed items cleared
  • Auto-subscribe users to push notifications when opening a list, with per-list opt-out toggle persisted in localStorage

Changes

Backend

• PushNotificationService: Rewrite buildPayload() with typed change actions (ITEM_ADDED, ITEM_CHECKED, ITEM_UNCHECKED, ITEM_UPDATED,
  ITEM_DELETED, ITEMS_CLEARED) and descriptive notification titles/bodies
• ListRoutes: Distinguish check/uncheck from text edit in update endpoint; fetch item text before deletion for notification context;
  read deviceId from query param instead of header

Frontend

• PushNotificationContext: Add autoSubscribe(), setOptedOut(), isOptedOut() to context; opt-out state stored per list in localStorage (
  shoppimo_push_optout_{listId})
• NotificationBell: Auto-subscribe on mount with useRef guard; click toggles opt-out flag before subscribe/unsubscribe; unsubscribed
  state shows gray styling
• Base64 fix: Convert btoa() output to URL-safe Base64 (+→-, /→_, strip =) to match backend's Base64.getUrlDecoder()