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/index.css

@@ -0,0 +1,27 @@
+@import "tailwindcss";
+
+body {
+  margin: 0;
+  font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
+  background: #0f172a;
+  color: #e2e8f0;
+}
+
+#root {
+  min-height: 100svh;
+}
+
+::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+::-webkit-scrollbar-track {
+  background: #1e293b;
+}
+::-webkit-scrollbar-thumb {
+  background: #475569;
+  border-radius: 4px;
+}
+::-webkit-scrollbar-thumb:hover {
+  background: #64748b;
+}