v2.16.0 - Habit tracking

Added

• Any task can now be tracked as a daily habit (#74). Tick "Track as a daily habit" when adding or editing a task, and it shows up on a new Habits view (sidebar link).
• Each habit on the Habits view shows:
  • a current streak - the number of consecutive days you've checked it in (counting back from today; not checking in today yet doesn't break the run until tomorrow)
  • a last-7-days dot row - one filled dot per day you checked in
  • a big "Mark done today" toggle - one tap logs today's check-in (tap again to undo). Checking in is idempotent: doing it twice in a day doesn't double-count.
• Claude can check a habit in for you over the MCP connector: a new log_habit_checkin tool ("I meditated today", "mark my workout done"), and create_task / update_task accept an isHabit flag.

Habits are decoupled from recurring tasks: a habit is an ongoing daily check-in, not a finish-once task. Check-ins are stored per day in a new table (a small database migration; existing tasks are untouched and default to not-a-habit). A GitHub-style calendar heatmap is a deliberate later addition - the check-in data already supports it.

This is the finale of the recurring + habits program (v2.13.0 comments -> v2.14.0 recurrence -> v2.14.1 advanced grammar -> v2.15.0 quick-add -> v2.16.0 habits).