feat(discussions): add GitHub Discussions activity widget

[mallya-m]

What does this PR do?

Adds a Discussion Activity widget to the dashboard showing how active the user is in GitHub Discussions — discussions started, comments given, and marked-as-answer count.

Related issue

Closes #258

Changes made

• New GET /api/metrics/discussions route using GitHub REST search + GraphQL
• REST: discussions started via /search/issues?q=author:{user}+type:discussion
• GraphQL: comments given and marked-as-answer count
• 5-minute Cache-Control header to avoid rate limit issues
• New DiscussionsWidget.tsx component with 3 stat chips
• Added widget to dashboard below Issue Metrics

How to test

1. Sign in and go to dashboard
2. Scroll to Discussion Activity widget
3. Three stat chips show: Discussions Started, Comments Given, Marked as Answer

Screenshots

[vercel]

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[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

[mallya-m]

@Priyanshu-byte-coder could you please add the gssoc:approved , intermediate , quality and type:feature labels?
Thank you!

[Priyanshu-byte-coder]

Please fix before merging:

1. Missing EOF newlines — all changed files must end with \n
2. Hardcoded Tailwind red colors in DiscussionsWidget — replace with CSS vars:
   • text-red-* → text-[var(--destructive)]
   • bg-red-* → bg-[var(--destructive)]

Colors must go through CSS variables to support dark/light theming.