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/SettingsTests.swift

@@ -11,34 +11,33 @@ final class SettingsTests: BaseUITestCase {
     override var seedFixture: String? { "empty" }
     override var bypassSubscription: Bool { false }
 
-    /// Navigate to Settings and verify the header and upgrade banner appear.
+    /// TC: Navigate to Settings and verify the header and upgrade banner appear.
     func testSettingsTab_ShowsHeaderAndUpgradeBanner() {
-        let tabBar = TabBarScreen(app: app)
-        let settingsScreen = tabBar.tapSettings()
+        let settingsScreen = TabBarScreen(app: app).tapSettings()
 
         settingsScreen.assertVisible()
-
-        // With subscription NOT bypassed, upgrade banner should be visible
         settingsScreen.assertUpgradeBannerVisible()
 
         captureScreenshot(name: "settings_with_upgrade_banner")
     }
 
-    /// Toggle between Customize and Settings segments.
+    /// TC: Toggle between Customize and Settings segments.
     func testSettingsTab_SegmentedControlToggle() {
-        let tabBar = TabBarScreen(app: app)
-        let settingsScreen = tabBar.tapSettings()
-
+        let settingsScreen = TabBarScreen(app: app).tapSettings()
         settingsScreen.assertVisible()
 
-        // Switch to Settings sub-tab
+        // Switch to Settings sub-tab and verify the segment exists
         settingsScreen.tapSettingsTab()
-        // Verify we're on the Settings sub-tab (check for a settings-specific element)
-        // The "Settings" segment should be selected now
+        settingsScreen.settingsSegment
+            .waitForExistenceOrFail(timeout: defaultTimeout, message: "Settings segment should exist after tapping it")
+
         captureScreenshot(name: "settings_subtab")
 
-        // Switch back to Customize
+        // Switch back to Customize and verify
         settingsScreen.tapCustomizeTab()
+        settingsScreen.customizeSegment
+            .waitForExistenceOrFail(timeout: defaultTimeout, message: "Customize segment should exist after tapping it")
+
         captureScreenshot(name: "customize_subtab")
     }
 }