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/MonthViewTests.swift

@@ -10,19 +10,15 @@ import XCTest
 final class MonthViewTests: BaseUITestCase {
     override var seedFixture: String? { "week_of_moods" }
 
-    /// TC-030: Navigate to Month view and verify content is visible.
+    /// TC-030: Navigate to Month view and verify the month grid is visible.
     func testMonthView_ContentLoads() {
         let tabBar = TabBarScreen(app: app)
         tabBar.tapMonth()
 
-        XCTAssertTrue(tabBar.monthTab.isSelected, "Month tab should be selected")
-
-        // Wait for month view content to load - look for any visible content
-        // Month cards should have mood color cells or month headers
-        let monthContent = app.scrollViews.firstMatch
-        XCTAssertTrue(
-            monthContent.waitForExistence(timeout: 5),
-            "Month view should have scrollable content"
+        let monthGrid = app.element(UITestID.Month.grid)
+        monthGrid.waitForExistenceOrFail(
+            timeout: navigationTimeout,
+            message: "Month grid should be visible after navigating to Month tab"
         )
 
         captureScreenshot(name: "month_view_with_data")
@@ -32,17 +28,17 @@ final class MonthViewTests: BaseUITestCase {
 final class MonthViewEmptyTests: BaseUITestCase {
     override var seedFixture: String? { "empty" }
 
-    /// TC-031: Navigate to Month view with no data - should not crash.
+    /// TC-031: Navigate to Month view with no data -- should not crash.
     func testMonthView_EmptyState_NoCrash() {
         let tabBar = TabBarScreen(app: app)
         tabBar.tapMonth()
 
-        XCTAssertTrue(tabBar.monthTab.isSelected, "Month tab should be selected")
-
-        // The view should load without crashing, even with no data.
-        // Give it a moment to render.
-        let monthTabStillSelected = tabBar.monthTab.waitForExistence(timeout: 3)
-        XCTAssertTrue(monthTabStillSelected, "App should not crash on empty month view")
+        // The month grid should still render even with no data
+        let monthGrid = app.element(UITestID.Month.grid)
+        monthGrid.waitForExistenceOrFail(
+            timeout: navigationTimeout,
+            message: "Month grid should render without crashing on empty data"
+        )
 
         captureScreenshot(name: "month_view_empty")
     }