add bio field to profiles

[saurabhhhcodes]

What changed

• Added a Bio/About field to profile settings with a live character counter.
• Persisted the bio through the profile API and user JWT payload.
• Stored the bio on the user record and surfaced it in the profile sidebar card.

Why

Users could edit their display name and avatar, but there was no place for a short profile bio.

Impact

Profiles feel more complete and the bio now survives saves, refreshes, and token-based app loads.

Validation

• git diff --check
• node --check server/src/routes/profile.js
• node --check server/src/lib/authJwtPayload.js
• node --check server/src/models/User.js
• npm run build in frontend

Closes #212

[vercel]

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[FelixDevX]

Hi! I was assigned this issue and have completed the implementation and local testing. Please let me know if any changes are needed. Thank you for your review.