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.

gscript/Automation.js

@@ -0,0 +1,33 @@
+function nightlyMaintenance() {
+  const cutoff = Date.now() - 14*3600000;
+  rows_(CFG.SHEETS.SHIFTS)
+    .filter(s => String(s.status)==='OPEN' && new Date(s.clock_in).getTime() < cutoff)
+    .forEach(s => {
+      append_(CFG.SHEETS.NOTIFICATIONS,{
+        notification_id:uuid_('NOT'),
+        user_id:'',
+        type:'OPEN_SHIFT_WARNING',
+        title:'Pravděpodobně chybí odchod',
+        message:'Směna '+s.shift_id+' je otevřená déle než 14 hodin.',
+        url:'',
+        read_at:'',
+        created_at:now_(),
+        email_sent_at:''
+      });
+    });
+}
+
+function monthlyMaintenance() {
+  const d = new Date();
+  d.setMonth(d.getMonth()-1);
+  const p = Utilities.formatDate(d,CFG.TZ,'yyyy-MM');
+  rows_(CFG.SHEETS.EMPLOYEES)
+    .filter(e => e.active === true || String(e.active).toUpperCase()==='TRUE')
+    .forEach(e => {
+      const exists = findOne_(CFG.SHEETS.MONTH_CLOSURES,r=>String(r.employee_id)===String(e.employee_id)&&String(r.period)===p);
+      if (!exists) append_(CFG.SHEETS.MONTH_CLOSURES,{
+        closure_id:uuid_('CLOSE'),employee_id:e.employee_id,period:p,
+        employee_confirmed_at:'',manager_approved_at:'',manager_approved_by:'',locked_at:'',status:'WAITING_EMPLOYEE'
+      });
+    });
+}