Add 4 new E2E tests for mobile viewport behavior

Tests mobile responsiveness including:
- Login page renders correctly on mobile viewport
- Dashboard displays correctly on mobile viewport
- Dashboard uses single-column layout on mobile (< 768px)
- Navigation elements are interactive on mobile

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

IMPLEMENTATION_PLAN.md

@@ -4,7 +4,7 @@ This file is maintained by Ralph. Run `./ralph-sandbox.sh plan 3` to generate ta
 
 ## Current Status: Feature Complete
 
-**Test Coverage:** 1014 unit tests (51 files) + 186 E2E tests (12 files) = 1200 total tests
+**Test Coverage:** 1014 unit tests (51 files) + 190 E2E tests (13 files) = 1204 total tests
 
 All P0-P5 items are complete. The project is feature complete.
 
@@ -97,7 +97,7 @@ All P0-P5 items are complete. The project is feature complete.
 | PeriodDateModal | 22 | Period input modal |
 | Skeletons | 29 | Loading states with shimmer |
 
-### E2E Tests (12 files, 186 tests)
+### E2E Tests (13 files, 190 tests)
 | File | Tests | Coverage |
 |------|-------|----------|
 | smoke.spec.ts | 3 | Basic app functionality |
@@ -112,6 +112,7 @@ All P0-P5 items are complete. The project is feature complete.
 | history.spec.ts | 7 | History page |
 | plan.spec.ts | 7 | Plan page |
 | health.spec.ts | 3 | Health/observability |
+| mobile.spec.ts | 4 | Mobile viewport behavior, responsive layout |
 
 ---
 
@@ -124,7 +125,6 @@ These are optional enhancements to improve E2E coverage. Not required for featur
 |------|-------|-------------|
 | notifications.spec.ts | 3 | Notification preferences |
 | dark-mode.spec.ts | 2 | System preference detection |
-| mobile.spec.ts | 4 | Mobile viewport behavior |
 
 ### Existing File Extensions
 | File | Additional Tests | Focus Area |
@@ -148,6 +148,7 @@ These are optional enhancements to improve E2E coverage. Not required for featur
 
 ## Revision History
 
+- 2026-01-13: Added mobile.spec.ts with 4 E2E tests (mobile viewport behavior, responsive layout)
 - 2026-01-13: Added 6 auth E2E tests (OIDC button display, loading states, session persistence across pages/refresh)
 - 2026-01-13: Added 5 settings persistence E2E tests (notification time, timezone, multi-field persistence)
 - 2026-01-13: Added 5 period-logging E2E tests (modal flow, future date restriction, edit/delete flows)