milestone: MVP — stable layout, all core features working

Reverted sticky header attempt back to clean two-column layout.
Server now sends no-cache headers for all static files.

Working features:
- People grid (18 TKB + 5 IT) with year-long calendar
- Shift codes (4/6/8/12/A/B/D/N/U/O) with auto-colored backgrounds
- Drag-and-drop cells (including color-only cells)
- Multi-cell selection (click/ctrl/shift/drag) with bulk operations
- Right-click context menu: set value, color palette, comments
- Info rows: TKB, Metro, D8 (toggleable)
- Czech holidays highlighted with names
- Cell value filter toggles in legend
- PDF export (2-page A4 landscape, Czech font support)
- Improvement proposals system
- Auto-contrast text color on manual backgrounds
- Opens at current month on load

TODO: sticky header (needs different approach)

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

web/server.js

@@ -374,10 +374,8 @@ app.get('/api/export-excel', (_req, res) => {
 
 // Serve static files from dist/
 app.use(express.static(join(__dirname, 'dist'), {
-  setHeaders: (res, path) => {
-    if (path.endsWith('.html')) {
-      res.setHeader('Cache-Control', 'no-cache, no-store, must-revalidate')
-    }
+  setHeaders: (res) => {
+    res.setHeader('Cache-Control', 'no-cache, no-store, must-revalidate')
   }
 }))