feat: add markdown support for public profile bio

[gitsofyash]

Summary

Closes #1075

Adds Markdown support for user bios on public profiles, including sanitized rendering, a live settings preview, and a 500-character bio limit.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Added sanitized Markdown rendering for public profile bios
• Added support for bold, italic, links, inline code, and line breaks
• Added a live Markdown preview toggle in the settings bio editor
• Added character count and 500-character validation
• Added Supabase schema and migration changes for the bio field

How to Test

Steps for the reviewer to verify this works:

1. Run npm run lint
2. Open /dashboard/settings
3. Add a bio using Markdown like **bold**, _italic_, [link](https://example.com), and inline code
4. Toggle the Markdown preview and verify the rendered output
5. Save the bio and open the public profile page
6. Verify links open in a new tab and HTML is sanitized

Screenshots (if UI change)

Not added.

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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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

[Priyanshu-byte-coder]

This PR has 12 merge conflicts with current main (across supabase.ts, settings routes, public profile, and the migration file name clashes). Please rebase onto current main and resolve all conflicts before this can be merged.

[Priyanshu-byte-coder]

Your PR has conflicts with the current main branch. Please rebase on main to resolve conflicts and this can be reviewed for merging. Labels have been added ✅

[gitsofyash]

Hi @Priyanshu-byte-coder, I rebased this PR onto the latest main again.

Current status:

• Branch is now up to date with main
• Merge conflicts are resolved
• CI passed
• Type Check passed

The remaining E2E failure is in e2e/notifications.spec.js where the test cannot find the Notifications button. This appears unrelated to the Markdown bio changes, since this PR only touches the profile bio/settings rendering path and schema/dependency files.

Could you please re-review when you get a chance?

[Priyanshu-byte-coder]

Hi! Your PR has merge conflicts with the current main branch. Please rebase your branch on latest main and resolve the conflicts:

git fetch upstream
git rebase upstream/main
# resolve any conflicts
git push --force-with-lease origin your-branch

Once resolved, we'll merge it in. Thanks!