🎨 A cleaner SubHub and a guided request flow

This is the largest update to SubHub so far. The whole interface got a calmer, more consistent look, and several rough edges in the daily flow are now smooth. Here's what changed for each role.

Requesting a sub used to be one dense form. It's now a short, guided wizard that takes you through dates, reason, subjects, and review one step at a time.

• Pick a single day or drag across a range with one date picker.
• Choose a reason (sick, personal, professional development, or other) as a real field, so it sticks with the request.
• Confirm which subjects the sub will cover, pre-checked for a full absence, so the common case is one tap.
• Mark a request urgent for a genuine emergency to move it to the front and reach subs by text first.

Admins can now select a stack of coverage requests and approve them in a single action. Each one is approved on its own, so a single problem row doesn't hold up the rest, and you get a clear summary like "8 approved, 2 couldn't be approved" with the reasons inline. A busy morning's queue clears in about four seconds.

The calendar now offers month, week, day, and agenda views, not just a single month grid. Switch with the toolbar toggle to match what you're looking for: the wide picture, a single week, one busy day by the hour, or a running chronological list.

Substitutes working several buildings in a week now have one place to prepare. The lesson plans page shows the plans for jobs you've claimed, grouped by school and sorted by date, so you can get ready the night before from your phone.

Paid time off is now measured consistently in hours across SubHub: a full day is eight hours, a half day is four. Your balance, the wizard's deduction, and the admin's adjustments all speak the same unit.

The refresh is built on a shared design system, so SubHub looks and behaves consistently from page to page, and dark mode now works throughout. Dead buttons that didn't do anything were removed rather than left to mislead.