Fix P3.10 documentation inconsistency

- Mark P3.10 E2E Test Suite as COMPLETE (was showing unchecked)
- Update file path from tests/e2e/ to e2e/ (actual location)
- Add reference to P5.4 which has full implementation details

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

IMPLEMENTATION_PLAN.md

@@ -694,10 +694,10 @@ Testing, error handling, and refinements.
   - Email includes days until expiry and instructions for refreshing tokens
 - **Why:** Users need time to refresh tokens (per spec requirement in specs/email.md)
 
-### P3.10: E2E Test Suite
-- [ ] Comprehensive end-to-end tests
+### P3.10: E2E Test Suite ✅ COMPLETE (See P5.4)
+- [x] Comprehensive end-to-end tests
 - **Files:**
-  - `tests/e2e/*.spec.ts` - Full user flows
+  - `e2e/*.spec.ts` - Full user flows (64 tests across 6 files)
 - **Test Scenarios:**
   - Login flow
   - Period logging and phase calculation
@@ -706,6 +706,7 @@ Testing, error handling, and refinements.
   - Garmin connection flow
   - Calendar subscription
 - **Why:** Confidence in production deployment
+- **Note:** Implementation details documented in P5.4
 
 ### P3.11: Missing Component Tests ✅ COMPLETE
 - [x] Add unit tests for untested components