Feature/coordinator availability UI

[martin0024]

Closes #130

Summary

Adds an Availability page where coordinators set when they're available for private lessons: weekly hours, time zone, date overrides and a preview
of the next few weeks. UI only. It builds on the backend from #119 / #124, plus one new read action.

Changes

Page & navigation

• New /availability page (app/(authenticated)/availability), using the same Suspense + async content pattern as children/page.tsx.
• Only coordinators can open it. Admins and regular users are redirected to /.
• Availability sidebar link, shown to coordinators only.

Editor (components/availability/)

Takes a coordinatorId prop, so the admin issue ("Admin — edit any coordinator's availabilities") can reuse it.

• Weekly hours: add, edit or remove time windows for each weekday. Each window repeats every week or every other week (with a start date).
• Time zone: picker in the Weekly hours card, saved together with the hours.
• Inline validation: overlapping windows, an end time before the start time, and a missing start date for every-other-week are flagged on the
  window itself, and Save is disabled. Any error from the server is shown in the card footer.
• Date overrides: a date picker dialog to set custom hours or a day off for one date, pre-filled with that day's usual hours. It lists upcoming
  overrides, including days off, with edit and remove.
• Preview: a 4-week calendar combining weekly hours and overrides. It marks overrides and days off and pages forward 4 weeks at a time.
• How bookable slots work: explains that each window is cut into back-to-back slots of the lesson's length and that leftover time can't be booked.
  Each window shows a hint like "3 × 60-min slots · last 30 min not bookable", based on the coordinator's active private lessons.
• The UI notes that changes only affect future bookings. Lessons already booked are never modified, since only the availability tables are written.

Backend

• New read action listCoordinatorAvailabilityOverrides({ coordinatorId, from }), which returns upcoming overrides including days off (windows: []).
  It uses the same permission checks as the other availability actions.
• The overlap check moved from app/private-lessons/schema.ts into lib/availability.ts (toMinutes, windowsConflict), so the server and the UI
  apply the same rule.
• lib/availability-editor.ts holds the page's pure helpers: time formatting, slot splitting, per-window validation, time zone labels and building the
  preview grid.

Tests

• __tests__/coordinator-availability-actions.test.ts: 5 new tests for listCoordinatorAvailabilityOverrides (returns days off, admin access, blocks
  regular users and other coordinators, rejects invalid dates).
• __tests__/availability-editor.test.ts: tests for formatting, slot splitting, validation (including every-other-week windows that overlap only on
  alternate weeks), date and time zone helpers, and the preview grid.
• registrations-view.test.tsx fails the same way on develop; that's unrelated to this PR.