Rewrite all UI tests following fail-fast TEST_RULES patterns

Rewrote 60+ test files to follow honeydue-style test guidelines:
- defaultTimeout=2s, navigationTimeout=5s — fail fast, no long waits
- No coordinate taps (except onboarding paged TabView swipes)
- No sleep(), no retry loops
- No guard...else { return } silent passes — XCTFail everywhere
- All elements by accessibility ID via UITestID constants
- Screen objects for all navigation/actions/assertions
- One logical assertion per test method

Added missing accessibility identifiers to app views:
- MonthView.swift: added AccessibilityID.MonthView.grid to ScrollView
- YearView.swift: added AccessibilityID.YearView.heatmap to ScrollView

Framework rewrites:
- BaseUITestCase: added session ID, localeArguments, extraLaunchArguments
- WaitHelpers: waitForExistenceOrFail, waitUntilHittableOrFail,
  waitForNonExistence, scrollIntoView, forceTap
- All 7 screen objects rewritten with fail-fast semantics
- TEST_RULES.md added with non-negotiable rules

Known remaining issues:
- OnboardingTests: paged TabView swipes unreliable on iOS 26 simulator
- SettingsLegalLinksTests: EULA/Privacy buttons too deep in DEBUG scroll
- Customization horizontal picker scrolling needs further tuning

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Tests iOS/DarkModeStylesTests.swift

@@ -14,21 +14,18 @@ final class DarkModeStylesTests: BaseUITestCase {
     func testDayViewStyles_DarkMode_NoCrash() {
         let tabBar = TabBarScreen(app: app)
         let customizeScreen = CustomizeScreen(app: app)
+        let dayScreen = DayScreen(app: app)
 
-        // First, switch to dark mode via the theme mode selector
+        // Switch to dark mode via theme mode selector
         let settingsScreen = tabBar.tapSettings()
         settingsScreen.assertVisible()
         settingsScreen.tapCustomizeTab()
 
-        // Try to select the "Dark" theme mode
-        let darkButton = customizeScreen.themeButton(named: "Dark")
-        if darkButton.waitForExistence(timeout: 3) || app.swipeUntilExists(darkButton, direction: .up, maxSwipes: 3) {
-            darkButton.tapWhenReady()
-        }
+        customizeScreen.selectTheme("Dark")
 
-        // Navigate to Day tab to verify dark mode renders correctly
+        // Verify Day tab renders in dark mode
         tabBar.tapDay()
-        assertDayContentVisible()
+        dayScreen.assertAnyEntryExists()
 
         captureScreenshot(name: "day_view_dark_mode_default_style")
 
@@ -43,7 +40,7 @@ final class DarkModeStylesTests: BaseUITestCase {
             customizeScreen.selectDayViewStyle(style)
 
             tabBar.tapDay()
-            assertDayContentVisible()
+            dayScreen.assertAnyEntryExists()
         }
 
         captureScreenshot(name: "day_view_dark_mode_styles_completed")