feat(notifications): add in-app notification bell for goals and streaks

[mallya-m]

What does this PR do?

Adds a notification bell icon to the dashboard header that shows unread
notification count and a dropdown list of recent notifications.

Related issue

Closes #232

Changes made

• Created notifications Supabase table with user_id, type, message, read, created_at
• New GET /api/notifications endpoint — returns 10 most recent + unread count
• New PATCH /api/notifications endpoint — marks all as read
• Created NotificationBell.tsx component with bell icon, unread badge, dropdown
• Polls every 60s for new notifications via setInterval in useEffect
• Marks all as read when dropdown is opened
• Added bell to DashboardHeader next to avatar

How to test

1. Sign in and go to dashboard
2. Bell icon appears in header next to avatar
3. Click bell — dropdown opens
4. Insert a test notification via Supabase SQL editor
5. Within 60s (or on refresh) badge appears with unread count
6. Click bell — notifications show, badge clears

Screenshots

[vercel]

@mallya-m is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[mallya-m]

@Priyanshu-byte-coder
Could you please add the gssoc:approved and type:feature labels?
Thank you!

[Priyanshu-byte-coder]

1. Missing migration file — PR description says it created the notifications table but no supabase/migrations/ file is in the diff. The API will throw on every request in any fresh deployment. Include the migration.

2. /notifications page doesn't exist — the "See all notifications" link points to a 404. Remove it or create the page.

3. Unrelated file changes — BadgeSection.tsx and CommitTimeChart.tsx are modified with changes unrelated to notifications (removing the Tip box, changing button colors). These belong in separate PRs.

4. Hardcoded text-white on the unread count badge — use text-[var(--accent-foreground)].

5. Missing EOF newlines on route.ts and NotificationBell.tsx.