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/YearViewHeatmapTests.swift

@@ -16,20 +16,10 @@ final class YearViewHeatmapTests: BaseUITestCase {
         let tabBar = TabBarScreen(app: app)
         tabBar.tapYear()
 
-        XCTAssertTrue(tabBar.yearTab.isSelected, "Year tab should be selected")
-
-        // Heatmap grid should be visible
         let heatmap = app.element(UITestID.Year.heatmap)
-        XCTAssertTrue(
-            heatmap.waitForExistence(timeout: 8),
-            "Year View heatmap grid should be visible with data"
-        )
-
-        // Stats section should also be visible (has data)
-        let statsSection = app.element(UITestID.Year.statsSection)
-        XCTAssertTrue(
-            statsSection.waitForExistence(timeout: 5),
-            "Year stats section should be visible"
+        heatmap.waitForExistenceOrFail(
+            timeout: navigationTimeout,
+            message: "Year View heatmap grid should be visible with data"
         )
 
         captureScreenshot(name: "year_heatmap_rendered")