feat(notifications): recipient-scoped delivery with mute and block preferences

[Varshinibhargav-17]

Summary

Implements recipient-scoped notifications with mute/block preferences,
fixing the architectural issues described in #359.

Problem

The existing notification system stored all notifications in a shared
flat collection with no recipient scoping. getNotificationData fetched
ALL notifications globally, read/delete operations used the flat
collection path, and the Block user UI button in NotificationBox.jsx
had no implementation.

Changes Made

Firestore structure:
Notifications are now stored at notifications/{uid}/userNotifications/{notification_id}
— one subcollection per user, fully recipient-scoped.

addNotification:

• Now accepts recipient_uid parameter
• Checks notification_preferences/{uid}.blockedUsers before creating
  notification — skips delivery if sender is blocked
• Stores recipient_uid and type fields in notification document

getNotificationData:

• Now accepts uid parameter
• Fetches only notifications/{uid}/userNotifications — no more
  global fetch

readNotification / deleteNotification:

• Updated to use subcollection path with uid

blockUser action (new):

• Writes username to notification_preferences/{uid}.blockedUsers
  via arrayUnion
• Future notifications from blocked user are silently skipped

muteUser action (new):

• Writes username to notification_preferences/{uid}.mutedUsers
  via arrayUnion

NotificationBox.jsx:

• Block button now calls blockUser action
• Added Mute button calling muteUser action
• Both use firebase.auth().currentUser?.uid for safety

dataReducer.js:

• Added blockedUsers and mutedUsers to initial state
• BLOCK_USER_SUCCESS filters blocked user's notifications from state
• MUTE_USER_SUCCESS adds to mutedUsers list

actionTypes.js:

• Added BLOCK_USER_START/SUCCESS/FAIL
• Added MUTE_USER_START/SUCCESS/FAIL
• Added GET_BLOCKED_USERS_SUCCESS

Testing

• npm run build — 0 errors
• npm run dev — app loads correctly
• Block button in notification menu now functional
• Mute button added alongside Block

Fixes #359