feat(frontend): build create bounty form

[Xuccessor]

Summary

Authenticated users (wallet connected) can post a new bounty.

Acceptance Criteria

• Route /bounties/new — redirect to home if not authenticated
• Form fields: title (required), description (markdown textarea), reward amount in XLM (required), deadline date picker
• Client-side validation with react-hook-form + zod
• On submit: POST /bounties with JWT in Authorization header
• On success: redirect to the new bounty's detail page
• On error: display field-level error messages