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/src/pages/AdminApp.tsx

@@ -1,8 +1,10 @@
 import { useEffect, useState } from 'react';
 import { EmployeeManager } from '../components/EmployeeManager';
 import { AdminStats } from '../components/AdminStats';
+import { ShiftPlanManager } from '../components/ShiftPlanManager';
+import { PayrollManager } from '../components/PayrollManager';
 
-type Tab = 'employees' | 'stats';
+type Tab = 'employees' | 'stats' | 'shifts' | 'payroll';
 
 export function AdminApp() {
   const [tab, setTab] = useState<Tab>('employees');
@@ -13,6 +15,8 @@ export function AdminApp() {
       if (target.tagName === 'INPUT' || target.tagName === 'TEXTAREA') return;
       if (e.key === '1') setTab('employees');
       if (e.key === '2') setTab('stats');
+      if (e.key === '3') setTab('shifts');
+      if (e.key === '4') setTab('payroll');
     }
     window.addEventListener('keydown', onKey);
     return () => window.removeEventListener('keydown', onKey);
@@ -30,10 +34,19 @@ export function AdminApp() {
         <button className={`tab ${tab === 'stats' ? 'active' : ''}`} onClick={() => setTab('stats')}>
           [2] Statistiky
         </button>
+        <button className={`tab ${tab === 'shifts' ? 'active' : ''}`} onClick={() => setTab('shifts')}>
+          [3] Směny
+        </button>
+        <button className={`tab ${tab === 'payroll' ? 'active' : ''}`} onClick={() => setTab('payroll')}>
+          [4] Mzdy
+        </button>
       </div>
 
       <div style={{ marginTop: '1rem' }}>
-        {tab === 'employees' ? <EmployeeManager /> : <AdminStats />}
+        {tab === 'employees' && <EmployeeManager />}
+        {tab === 'stats' && <AdminStats />}
+        {tab === 'shifts' && <ShiftPlanManager />}
+        {tab === 'payroll' && <PayrollManager />}
       </div>
     </>
   );