Add shift planning, month closure + payroll, and a dark modern redesign

Ports two modules from the friend's Google Apps Script build (kept as
reference in gscript/) onto the TypeScript stack, rewritten cleanly against
this app's own data model rather than copied 1:1:

- Shift planning: weekly template (Sun-Thu evening, Fri/Sat two slots),
  lazy idempotent generation per period (no cron needed), employee
  signup/cancel with collision + capacity checks, admin calendar view with
  slot editing and manual assignment.
- Month closure + payroll: employee confirms the month (blocked while a
  shift is still open), admin locks and finalizes pay (base hours * rate +
  tips/bonus/other), reopen to undo a premature lock, mark paid. Pay rates
  are versioned by date, defaulting the first-ever rate to apply
  retroactively to the employee's whole history.
- A shift left open more than 12h (forgotten clock-out) is auto-closed at
  clock_in + 12h, checked lazily on read instead of a background job.
- Full dark, sharp-edged modern restyle (theme.css replaces tui.css) with
  an amber accent, keeping every existing class name so no component
  logic needed to change.

Backend test coverage (jest) for all three workflows: shift planning,
closure/payroll, and the forgotten-clock-out auto-close.

frontend/vite.config.ts

@@ -13,8 +13,8 @@ export default defineConfig({
         name: 'EatMe — Docházka',
         short_name: 'EatMe',
         description: 'Evidence docházky zaměstnanců bistra EatMe',
-        theme_color: '#000000',
-        background_color: '#f0f0f0',
+        theme_color: '#0d0f13',
+        background_color: '#0d0f13',
         display: 'standalone',
         start_url: '/',
         icons: [