Add Twitter/X Handle to Frontend Profiles

Closes #173

Changes

Adds Twitter/X handle support to the frontend, mirroring the existing GitHub handle UX exactly.

New file:

• src/components/ui/XIcon.tsx — X (Twitter) SVG icon component

Type definitions:

• src/lib/types/api.d.ts — Added twitter_handle to CreateProfileInput and UpdateProfileInput
• src/lib/types/profiles.d.ts — Added twitterHandle to Profile, twitter_handle to ProfileFromAPI

Display:

• ProfileHeader — Shows X icon + clickable @handle link (links to https://x.com/{handle})
• ProfileMain / ProfileActions — Thread twitterHandle prop through component tree

Forms:

• EditProfileDialog / CreateProfileDialog — Added "Twitter/X Handle" form field with @ prefix, matching GitHub Handle field UX

List view:

• ProfileCard / ProfilesList — Map and forward twitterHandle from API response

Mock data:

• profileConstants.ts — Added sample twitterHandle values

Testing

• Zero TypeScript errors across all modified files
• Same UX pattern as GitHub handles — icon, link, form field with @ prefix