refactor: organize notification classes with cn

[saurabhhhcodes]

Summary

• refactors the notification dropdown unread-state classes to use the existing cn helper
• moves notification type gradient classes into a typed lookup map instead of an inline nested ternary
• replaces the loose notification timestamp type with Firestore Timestamp while touching the component

Closes #129

Validation

• npx eslint src/components/NotificationDropdown.tsx
• git diff --check
• npm run build compiled successfully, then stopped during prerender because local Firebase env keys are not configured (auth/invalid-api-key)

Suggested GSSoC labels

gssoc:approved, level:beginner, type:refactor, quality:clean

[saurabhhhcodes]

Thanks for merging this. For GSSoC scoring, could you please add the labels when you get a chance? Suggested labels: gssoc:approved, level:beginner, type:refactor, quality:clean.

[saurabhhhcodes]

Thanks for merging this cleanup. This refactor keeps the notification styling consistent through the shared cn helper, so could you please add the missing GSSoC scoring labels when you get a chance? I think gssoc:approved, level:beginner, and type:refactor fit the actual change.

[Aditya948351]

Yeah adding those labels @saurabhhhcodes !