feat: implement bet slip summary, oracle circuit breaker, and dispute UI

[Fahmedo]

Summary

Closes #582, Closes #609, Closes #591 Closes #587

Replaces #646 (had merge conflicts with main).

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#591 — Bet Slip Summary Before Freighter Signing

• BetSlipSummary full-screen overlay with plain-language bet details
• All amounts computed from stroop integers (no floating point)
• Slippage warning when odds drift > 50 bps since amount entry
• Confirm & Sign triggers Freighter; Back returns to bet form without state loss
• Unit tests covering all field calculations, slippage detection, and button interactions

#587 — Oracle Circuit Breaker for Silent Backend Failures

• Consecutive failure counter increments on every failed cycle
• Admin webhook alert after 3 consecutive failures
• Circuit breaker trips at 10 failures: oracle interval paused, critical alert sent
• Counter resets to 0 on any successful cycle
• GET /api/health/oracle endpoint pinged before each cycle
• Unit tests covering failure counting, alert trigger, and circuit breaker

#557 — Dispute Submission UI for Market Outcomes

• POST /api/markets/:id/dispute endpoint with resolution history logging
• GET /api/markets/:id/dispute-status endpoint
• DisputeModal with reason validation (min 50 chars) and optional evidence URL
• DisputeStatusTracker showing Submitted → Under Review → Resolved
• Duplicate dispute prevention (button disabled after submission)

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