Stage 3: Fix bypass call sites and tab navigation intensity

[ElectricCookie]

Stage 3 of #139

Route the 3 raw HapticFeedback bypass calls through LdHaptics.emit and fix wrong intensity in tab navigation.

Files to modify

• lib/src/button.dart:217–219 — replace HapticFeedback.heavyImpact() × 2 with await LdHaptics.emit(context, LdHapticsEvent.buttonError) × 2 (200ms gap preserved in caller); remove HapticFeedback usage from this call path
• lib/src/date_picker.dart:194 — replace HapticFeedback.selectionClick() with LdHaptics.emit(context, LdHapticsEvent.selection)
• lib/src/appbar/tab_navigation.dart:157 — replace LdHaptics.vibrate(HapticsType.heavy) with LdHaptics.emit(context, LdHapticsEvent.indicatorDrag) (backend maps to selection-class, fixing the wrong-intensity bug)
• lib/src/appbar/tab_navigation.dart:169 — replace LdHaptics.vibrate(HapticsType.light) with LdHaptics.emit(context, LdHapticsEvent.selection)

Testing

• Widget test per modified component with injected MockHapticsBackend via LdThemeProvider(hapticsBackend: mock)
• Verify exact LdHapticsEvent fired for each gesture
• Verify zero raw HapticFeedback calls: grep -r 'HapticFeedback\.' lib/ --include='*.dart' must return empty

Dependencies

Depends on Stage 1 (#140) and Stage 2 (#141)