feat: mission create UI (web/create-mission.html)

[minhchee]

Task #24 — Web UI for posting missions (no-code form)

Adds web/create-mission.html — a self-contained, no-backend single-page app that lets anyone post a paid AIGEN mission through the documented REST API.

What it does

• Form for all POST /missions/create fields: creator_agent_id, title, description, reward_amount, reward_currency (AIGEN/USDC/ETH), reward_chain, verification_type (peer_vote / first_valid_match / creator_judges), accept_regex, deadline_hours, min_submitter_elo.
• Live payout quote via GET /missions/quote-payout (shows net to winner + 0.5% protocol fee).
• On submit: POST /missions/create. AIGEN missions go live immediately; USDC/ETH missions show funding_instructions with a deposit address and a client-side QR code.
• Auto-detects funding: polls GET /missions/{id} and flips status to active when funded (manual tx-hash confirm also supported).
• Pure static HTML+JS, dark AIGEN theme (#5fe8a3 accent), no backend changes. Drop-in next to the existing web/dashboard.html.

Verification

• Inline JS passes node --check.
• Calls only documented endpoints (/missions/create, /missions/quote-payout, /missions/{id}, /missions/{id}/confirm-funding).
• Self-contained: only external dependency is the qrcodejs CDN, with a graceful text fallback if it fails to load.

Context

Implements Task #24 — Web UI for posting missions (no-code form) from tasks.json.