Add PWA native feel: manifest, meta tags, install prompt, SW optimization, dynamic theme-color

Configure manifest.json with scope, maskable icons, and categories. Add iOS/Android
meta tags for standalone behavior. Create pwa.css for native touch/scroll handling
and safe area insets. Add oikos-install-prompt Web Component with Chrome install
flow and iOS guidance. Optimize service worker with network-first navigation and
expanded precache (v19). Add dynamic theme-color per route and modal overlay dimming
in standalone mode. Generate placeholder icons via sharp script.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -19,7 +19,7 @@
   "dependencies": {
     "bcrypt": "^5.1.1",
     "better-sqlite3": "^9.6.0",
-"dotenv": "^16.4.7",
+    "dotenv": "^16.4.7",
     "express": "^4.21.2",
     "express-rate-limit": "^7.5.0",
     "express-session": "^1.18.1",
@@ -33,5 +33,8 @@
   "license": "MIT",
   "engines": {
     "node": ">=20.0.0"
+  },
+  "devDependencies": {
+    "sharp": "^0.34.5"
   }
 }