feat: add "Mark as Solved" on Question posts with accepted answer flow

[MaheshN1821]

What this PR does

Adds a "Mark as Solved" feature for Question-type posts, allowing the
post author to accept a specific comment as the answer.

Changes

• app/dashboard/page.js — added handleMarkSolved handler that toggles
  solved and solvedCommentId fields on the post document in Firestore.
  Fires a "solved" notification to the comment author when accepted.

• components/dashboard/PostCard.js — added ✓ Accept button on each
  comment (visible only to the post author on Question posts). Accepted
  comment gets a green highlight and "✓ Accepted Answer" label. Post card
  header shows a green "✓ SOLVED" badge when resolved.

• components/dashboard/FeedColumn.js — forwards onMarkSolved prop
  through to PostCard.

• components/Notifications.js — added "solved" notification type so
  the comment author gets notified when their answer is accepted.

Flow

1. Author posts a Question
2. Community replies via comments
3. Author sees a ✓ Accept button next to each comment
4. Clicking it marks that comment as the accepted answer and adds a
   green ✓ SOLVED badge to the post
5. The action is toggleable — clicking again unmarks it
6. Comment author receives an in-app notification

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[ARUNNG2004]

@MaheshN1821 please add the screenshot images

[MaheshN1821]