feat: track habits with daily check-ins and streaks

[AmrMsCLL]

Stacked on #48. This is 11f of as2026-10-05-strata-phase-11-next-batch-2.md.

What changes

• Each space has a Habits tab. Editors add, rename and delete habits. A habit's goal is every day, or 1 to 6 days a week.
• Every member checks in for themselves, today or yesterday, in their own time zone. Viewers can check in too, since a check-in is only their own record.
• Streaks count consecutive days for daily habits, and consecutive weeks that met the goal for weekly ones. The current week counts once its goal is met. Each row shows the last seven days.
• New Habits Home widget (habits) lists your habits across spaces, with the same check-in controls.

Migration

20261005140000_habits creates habits (id, space_id, name, per_week, position, created_at) and habit_check_ins (habit_id, user_id, day, created_at, primary key on all three). Both cascade from spaces, habits and users.

API

• GET/POST /spaces/:id/habits, GET /me/habits, PATCH/DELETE /habits/:id
• PUT/DELETE /habits/:id/check-ins/:day
• Dashboard widget types gain habits.

Verification

• Server: test/habit-streaks.spec.ts (3 passed) and test/integration/habits.spec.ts (1 passed).
• Frontend: test/habits.spec.tsx (2 passed) and test/routes.spec.tsx.
• Lint and build pass. Checked on the local stack: adding habits and checking in for today and yesterday.