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/Database.js

@@ -0,0 +1,72 @@
+function db_() {
+  const id = PropertiesService.getScriptProperties().getProperty('DB_SPREADSHEET_ID');
+  if (!id) throw new Error('Databáze není inicializovaná. Spusť setupSystem().');
+  return SpreadsheetApp.openById(id);
+}
+
+function sh_(name) {
+  const s = db_().getSheetByName(name);
+  if (!s) throw new Error('Chybí list ' + name);
+  return s;
+}
+
+function rows_(name) {
+  const s = sh_(name);
+  const values = s.getDataRange().getValues();
+  if (values.length < 2) return [];
+  const headers = values[0].map(String);
+  return values.slice(1).filter(r => r.some(v => v !== '')).map(r => {
+    const o = {};
+    headers.forEach((h, i) => o[h] = r[i]);
+    return o;
+  });
+}
+
+function append_(name, obj) {
+  const s = sh_(name);
+  const headers = s.getRange(1,1,1,s.getLastColumn()).getValues()[0];
+  s.appendRow(headers.map(h => obj[h] !== undefined ? obj[h] : ''));
+  return obj;
+}
+
+function updateBy_(name, key, value, patch) {
+  const s = sh_(name);
+  const data = s.getDataRange().getValues();
+  if (!data.length) return false;
+  const headers = data[0].map(String);
+  const keyCol = headers.indexOf(key);
+  if (keyCol < 0) throw new Error('Sloupec ' + key + ' neexistuje v ' + name);
+  const row = data.slice(1).findIndex(r => String(r[keyCol]) === String(value));
+  if (row < 0) return false;
+  Object.keys(patch).forEach(k => {
+    const c = headers.indexOf(k);
+    if (c >= 0) s.getRange(row + 2, c + 1).setValue(patch[k]);
+  });
+  return true;
+}
+
+function findOne_(name, predicate) {
+  return rows_(name).find(predicate) || null;
+}
+
+function uuid_(prefix) {
+  return (prefix || 'ID') + '_' + Utilities.getUuid().replace(/-/g,'').slice(0,16).toUpperCase();
+}
+
+function now_() { return new Date(); }
+
+function isoDate_(d) {
+  return Utilities.formatDate(new Date(d), CFG.TZ, 'yyyy-MM-dd');
+}
+
+function period_(d) {
+  return Utilities.formatDate(new Date(d), CFG.TZ, 'yyyy-MM');
+}
+
+function emailNorm_(email) {
+  return String(email || '').trim().toLowerCase();
+}
+
+function json_(v) {
+  try { return JSON.stringify(v); } catch(e) { return String(v); }
+}