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/index.html

@@ -5,7 +5,7 @@
     <link rel="icon" type="image/x-icon" href="/favicon.ico" />
     <link rel="apple-touch-icon" href="/apple-touch-icon.png" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
-    <meta name="theme-color" content="#000000" />
+    <meta name="theme-color" content="#0d0f13" />
     <meta name="description" content="Evidence docházky zaměstnanců bistra EatMe" />
     <title>EatMe — Docházka</title>
     <script src="https://accounts.google.com/gsi/client" async defer></script>