[FEAT] Support reusable Week Templates ("Choose template") #160

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

Users often alternate between distinct seasonal or cyclical routines (e.g., "School/University Routine", "Exam Session Routine", "Summer/Vacation Routine"). The current "Copy Previous Week" feature only works if the immediately preceding week matches the desired routine, forcing users to recreate schedules from scratch when transitions happen.

Describe the solution you'd like

1. Apply Template:
   • On an empty week in PlannerScreen.tsx, display a secondary button below "Copy previous week":
     • [Copy Previous Week]
     • — or —
     • [Choose Template] (with an icon)
   • Tapping "Choose Template" opens a sheet/modal listing saved week templates (e.g., "School Week", "Summer Week").
   • Selecting a template populates the week with the saved blocks and handles calendar sync.
2. Save Current Week as Template:
   • Provide an option (e.g., in a menu or header action) to "Save Week as Template", allowing the user to provide a template name.
3. Template Management:
   • Allow users to view and delete created week templates.

Describe alternatives you've considered

Allowing users to navigate back to any past week and copy that specific week, but remembering which past week had which schedule is confusing and hard to maintain.

Additional Context

• Keeps data local-first (stored in AsyncStorage, e.g. @climbstack/week_templates).
• Maintains ClimbStack's minimal, native UI philosophy.
• Related files: src/screens/PlannerScreen.tsx, src/store/PlannerContext.tsx, src/types/index.ts.
• AI Generation Disclosure: This issue description was generated by an AI agent (Gemini 3.1 Pro).