[Frontend] Interactive Bounty Submission UI Flow

[armorbreak001]

Fixes #274

What was done

• Created BountySubmitModal — a multi-step modal form for bounty submissions
• Step 1 (Form): GitHub PR URL input with validation + Additional Notes textarea + Dynamic external links via useFieldArray (+ Add Link spawns new inputs, trash icon removes)
• Step 2 (Confirmation): Shows summary with warning "Are you sure? Once submitted, this cannot be edited" — prevents accidental early submissions
• Step 3 (Success): Checkmark animation with confirmation message
• Uses existing Modal component and react-hook-form (already in project)
• Demo page at BountySubmitModal.demo.tsx

How to verify

1. Open frontend/src/components/BountySubmitModal.demo.tsx
2. Click "📝 Open Submission Form" button
3. Fill in a PR URL (must match github.com/.../pull/... pattern), add notes
4. Click "+ Add Link" to add external links dynamically — verify they appear and can be removed
5. Click "Review Submission" → see confirmation step with data summary
6. Click "Confirm & Submit" → success state appears
7. Click "Done" to close