feat(pwa): add PWA manifest, window controls overlay, and offline caching support

[anmol-virk]

Summary #88

Adds PWA support to DATEX Workbench, making it installable as a desktop app.

Changes

• Added vite-plugin-pwa with autoUpdate service worker
• PWA manifest with display_override: window-controls-overlay so the app header fills the full title bar
• Offline/caching support via Workbox, caching all JS, CSS, HTML, fonts and images
• Updated HeaderProvider.vue with WCO CSS env variables so header respects titlebar area
• Updated App.vue to offset main content by titlebar height
• Added theme-color meta tag to index.html

How to test

1. npm run build && npm run preview
2. Open in Chrome → look for install icon in address bar
3. Install the app → header should fill the title bar with OS window controls overlaid on top
4. Go offline → app should still load from cache

##Related issue
Closes #25