feat: TKB shift scheduler — personnel shift planning web app

Full rewrite of METRO HMG for TKB tunnel department:
- People-based grid (18 TKB + 5 IT), year-long calendar
- Color-coded shift values (4/6/8/12/A/B/D/N/U/O)
- Drag-and-drop cells, multi-cell selection (click/ctrl/shift/drag)
- Right-click context menu with color palette
- Tunnel closure + Metro + D8 info rows (toggleable)
- Czech holidays highlighted with names
- PDF export (2-page A4 landscape, DejaVu font for Czech chars)
- Improvement proposals system
- Sticky headers (vertical + horizontal scroll)
- Cell value filter toggles in legend

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/src/Login.tsx

@@ -0,0 +1,83 @@
+import { useState } from 'react'
+
+const VALID_USER = 'tkb'
+const VALID_PASS = 'sluzby'
+const AUTH_KEY = 'tkb_auth'
+
+export function isAuthenticated(): boolean {
+  return sessionStorage.getItem(AUTH_KEY) === 'true'
+}
+
+export function setAuthenticated(): void {
+  sessionStorage.setItem(AUTH_KEY, 'true')
+}
+
+interface LoginProps {
+  onLogin: () => void
+}
+
+export function Login({ onLogin }: LoginProps) {
+  const [username, setUsername] = useState('')
+  const [password, setPassword] = useState('')
+  const [error, setError] = useState(false)
+
+  const handleSubmit = (e: React.FormEvent) => {
+    e.preventDefault()
+    if (username === VALID_USER && password === VALID_PASS) {
+      setAuthenticated()
+      onLogin()
+    } else {
+      setError(true)
+      setTimeout(() => setError(false), 3000)
+    }
+  }
+
+  return (
+    <div className="min-h-screen bg-slate-900 flex items-center justify-center">
+      <form
+        onSubmit={handleSubmit}
+        className="bg-slate-800 border border-slate-700 rounded-lg p-8 w-80 shadow-xl"
+      >
+        <h1 className="text-lg font-semibold text-slate-100 mb-1 text-center">
+          TKB Plan sluzeb
+        </h1>
+        <p className="text-sm text-slate-400 mb-6 text-center">
+          Planovani smen a pohotovosti
+        </p>
+
+        <label className="block text-xs text-slate-400 mb-1">Uzivatel</label>
+        <input
+          type="text"
+          value={username}
+          onChange={e => setUsername(e.target.value)}
+          className="w-full mb-4 px-3 py-2 rounded bg-slate-900 border border-slate-600 text-slate-100
+                     text-sm focus:outline-none focus:border-blue-500"
+          autoFocus
+        />
+
+        <label className="block text-xs text-slate-400 mb-1">Heslo</label>
+        <input
+          type="password"
+          value={password}
+          onChange={e => setPassword(e.target.value)}
+          className="w-full mb-6 px-3 py-2 rounded bg-slate-900 border border-slate-600 text-slate-100
+                     text-sm focus:outline-none focus:border-blue-500"
+        />
+
+        {error && (
+          <div className="mb-4 text-xs text-red-400 text-center">
+            Nespravne prihlasovaci udaje
+          </div>
+        )}
+
+        <button
+          type="submit"
+          className="w-full py-2 rounded bg-blue-600 hover:bg-blue-500 text-white text-sm
+                     font-medium transition-colors cursor-pointer"
+        >
+          Prihlasit
+        </button>
+      </form>
+    </div>
+  )
+}