feat(gotjunk): Multi-device cart reservation with 5-min timeout

[Foundup]

Summary

Implemented cross-device cart coordination with 5-minute reservation timeout. When user swipes right on Browse, item is reserved for 5 minutes and hidden from ALL users' Browse feeds. Countdown timer displays in cart fullscreen view. On expiration, item automatically returns to Browse feed across all devices.

Architecture

Reservation Flow:

1. Right Swipe → Creates CartReservation { reservedBy, reservedAt, expiresAt }
2. Firestore Sync → updateCartReservation(itemId, reservation)
3. Browse Filter → Hide items where isReservedByOthers() === true
4. Countdown Timer → MM:SS display in cart fullscreen (color-coded: green→yellow→red)
5. Auto-Expiration → After 5 minutes, item returns to Browse feed on all devices

Multi-Device Coordination:

• Phone A swipes right → Firestore reservation created
• Phone B sees item disappear from Browse (real-time)
• 5 minutes pass → expiration handler clears reservation
• Both devices sync → item reappears in Browse feeds

Files Changed

Created:

• frontend/services/cartReservation.ts (146 lines) - Service module following WSP 49 pattern
• frontend/components/CartCountdown.tsx (76 lines) - Color-coded countdown timer component

Modified:

• frontend/types.ts - Added CartReservation interface
• frontend/services/firestoreSync.ts - Added updateCartReservation()
• firestore.rules - Added isValidReservationUpdate() validator
• frontend/App.tsx - Right-swipe handler, browse filter, countdown integration, expiration handler
• ModLog.md - Documentation

Testing

• Build successful: ✓ npm run build (962.69 KB bundle)
• TypeScript compilation: ✓ No errors
• Multi-device testing: Ready (requires deployment)

Test Plan:

1. Open Phone A + Phone B (different devices or incognito tabs)
2. Capture item on Phone A, swipe right on Phone B's Browse
3. Verify item disappears from Phone A's Browse feed
4. Open cart fullscreen on Phone B → verify countdown timer
5. Wait 5 minutes → verify item returns to Browse on both devices

WSP Compliance

• WSP 49: Service module pattern (cartReservation.ts)
• WSP 98: Firestore cross-device coordination (Layer 2 - Global Index)
• WSP 50: Pre-action verification (HoloIndex search before implementation)
• WSP 22: ModLog documentation

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