feat(pwa): installable app + fix research tree pip overflow

Research tree UI:
- The level pips shared the header row with the upgrade name and fought
  for width; long names like "Festungsmauern" (5 pips) pushed the pips
  out of the upgrade card. Pips now sit in the bottom row next to the
  buy button (space-between), where width is plentiful, and carry a
  "level / max" tooltip
- Tighter modal padding on phones (max-width 560px), scrollable at 92vh

PWA (installable as app in mobile browsers):
- public/manifest.webmanifest: standalone display, any orientation,
  TRXTD theme colors, German lang, start_url "/"
- public/sw.js: network-first service worker with runtime caching.
  index.html is always fetched fresh so new deploys are picked up
  immediately; the cache only serves as offline fallback. API calls
  and cross-origin requests are never intercepted
- Icon set generated by scripts/generate-icons.mjs (pure Node PNG
  encoder, zero image dependencies): gold tower on the game-themed
  dark gradient in 192px, 512px, maskable 512px (motif inside the safe
  zone) and 180px apple-touch-icon
- index.html: manifest link, favicon, apple-touch-icon, theme-color,
  mobile-web-app-capable, apple-mobile-web-app meta tags
- src/main.ts: service worker registration in production builds only
  (dev HMR stays untouched)
- server.mjs: serve .webmanifest as application/manifest+json (Chrome
  requires the correct MIME type for installability)

Verified against the production server: manifest (application/
manifest+json), icons (image/png) and sw.js (text/javascript) respond
with 200, index.html references all PWA tags; npm test 48/48 green.

server/server.mjs

@@ -447,6 +447,7 @@ const MIME = {
   '.js': 'text/javascript; charset=utf-8',
   '.css': 'text/css; charset=utf-8',
   '.json': 'application/json; charset=utf-8',
+  '.webmanifest': 'application/manifest+json; charset=utf-8',
   '.png': 'image/png',
   '.jpg': 'image/jpeg',
   '.jpeg': 'image/jpeg',