[FEAT] Add "Clear Week" action with confirmation warning #159

Is your feature request related to a problem? Please describe.

When planning a week or after accidentally copying the previous week, clearing scheduled blocks requires opening each block individually and deleting it one by one. This causes unnecessary friction and frustration when resetting a week schedule.

Describe the solution you'd like

1. Add a Clear Week action button in PlannerScreen.tsx (visible when the current week has at least one scheduled block).
2. When tapped, trigger a native confirmation dialog:
   • Title: "Clear entire week?"
   • Message: "Are you sure you want to remove all scheduled blocks for this week? This action cannot be undone."
3. Upon confirmation, execute a clearCurrentWeek() context method that:
   • Clears all blocks for the active currentWeekId.
   • Properly removes all linked Google Calendar events via deleteCalendarEvent().
   • Persists the empty state to storage and updates the UI immediately.

Describe alternatives you've considered

Multi-select block deletion, but that adds significant UI complexity compared to a single, clean "Clear Week" button.

Additional Context

• Related files: src/screens/PlannerScreen.tsx, src/store/PlannerContext.tsx, src/utils/i18n.ts.
• AI Generation Disclosure: This issue description was generated by an AI agent (Gemini 3.1 Pro).