Add repeating tasks feature to Phase 7

Added detailed repeating tasks functionality to Advanced Features:

How it works:
- When a repeating task is completed, it automatically:
  1. Returns to backlog status
  2. Updates due date by specified interval

Supported intervals:
- daily, weekly, monthly, yearly
- Custom intervals (e.g., "every 3 days")
- Optional end date or repetition limit

Frontmatter fields:
- repeat: "daily" | "weekly" | "monthly" | "yearly" | "every N days/weeks/months"
- repeat_until: (optional) end date for repetitions

Example use cases:
- Daily standup notes
- Weekly review tasks
- Monthly bill payments
- Quarterly reports

This is a Phase 7 feature as it requires background processing
to check completed tasks and reset them based on intervals.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

PLAN.md

@@ -739,7 +739,12 @@ mod android {
 - [ ] Easy onboarding for Google Tasks users
 
 #### Advanced Task Management
-- [ ] Recurring tasks (daily, weekly, monthly, custom)
+- [ ] **Repeating tasks** (tasks that automatically uncomplete and reschedule)
+  - When completed, task automatically returns to backlog
+  - Due date updates by specified interval (e.g., +1 day, +1 week, +1 month)
+  - Intervals: daily, weekly, monthly, yearly, custom (e.g., "every 3 days")
+  - Optional: limit number of repetitions or end date
+  - Stored in frontmatter: `repeat: "daily"`, `repeat_until: "2026-01-01"`
 - [ ] Task templates (save common tasks)
 - [ ] Bulk operations (select multiple, bulk edit)
 - [ ] Full-text search across all tasks