Add 2 E2E tests for dark mode system preference detection

Verifies that the app correctly applies light/dark mode styling based on
the user's system preference (prefers-color-scheme). Tests cover both
light and dark modes using Playwright's emulateMedia API.

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) + 202 E2E tests (13 files) = 1216 total tests
+**Test Coverage:** 1014 unit tests (51 files) + 204 E2E tests (14 files) = 1218 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 (13 files, 202 tests)
+### E2E Tests (14 files, 204 tests)
 | File | Tests | Coverage |
 |------|-------|----------|
 | smoke.spec.ts | 3 | Basic app functionality |
@@ -113,6 +113,7 @@ All P0-P5 items are complete. The project is feature complete.
 | plan.spec.ts | 7 | Plan page |
 | health.spec.ts | 3 | Health/observability |
 | mobile.spec.ts | 7 | Mobile viewport behavior, responsive layout, calendar mobile |
+| dark-mode.spec.ts | 2 | System preference detection, light/dark theme application |
 
 ---
 
@@ -124,11 +125,11 @@ These are optional enhancements to improve E2E coverage. Not required for featur
 | File | Tests | Description |
 |------|-------|-------------|
 | notifications.spec.ts | 3 | Notification preferences |
-| dark-mode.spec.ts | 2 | System preference detection |
 
 ### Completed Enhancements
 | File | Tests Added | Focus Area |
 |------|-------------|------------|
+| dark-mode.spec.ts | +2 | System preference detection (light/dark mode) |
 | garmin.spec.ts | +4 | Network error recovery (save, disconnect, status fetch, retry) |
 | calendar.spec.ts | +4 | Accessibility (ARIA, keyboard nav) |
 | settings.spec.ts | +1 | Error recovery on failed save |
@@ -149,6 +150,7 @@ These are optional enhancements to improve E2E coverage. Not required for featur
 
 ## Revision History
 
+- 2026-01-13: Added dark-mode.spec.ts with 2 E2E tests (system preference detection for light/dark mode)
 - 2026-01-13: Added 4 Garmin E2E tests (network error recovery for save, disconnect, status fetch, retry)
 - 2026-01-13: Added 8 E2E tests (calendar accessibility, settings error recovery, calendar mobile behavior)
 - 2026-01-13: Added mobile.spec.ts with 4 E2E tests (mobile viewport behavior, responsive layout)