add search functionality to notifications drawer

[bhavyanjain3004]

Summary:

Added real-time search functionality to the Notification drawer (Closes #1080).

Key Changes:

Added a debounced search input inside the NotificationBell dropdown.
Notifications filter in real-time client-side.
Matching text in notifications is highlighted.
Includes an empty state message and a clear (×) button.
Input autofocuses on open for better accessibility.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[vercel]

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