feat: add AppContext refresh methods and notifications mark-as-read

[yusuftomilola]

Summary

• Adds refreshPosts() and refreshPostDetail(postId) to AppContextType contract and implements both in AppProvider — fetches from /api/posts and /api/posts/:id respectively and reconciles local state against server responses
• Updates the notifications page to add per-notification "Mark as read" buttons and a bulk "Mark all as read" action that call the existing PATCH /api/notifications/:id endpoint and update unread count in local state without a full reload
• Adds a typed Notification interface to the page and a 'use client' directive for Next.js client component compatibility

closes #281
closes #282

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