feat: PWA manifest + service worker for offline and install support

Adds full Progressive Web App support to The Nexus:

- sw.js: Service worker with cache-first strategy for local assets
  and stale-while-revalidate for CDN resources (Three.js, fonts)
- offline.html: Styled offline fallback page with auto-reconnect
- icons/nexus-icon.svg: Nexus crystal sigil icon (SVG)
- icons/nexus-maskable.svg: Maskable icon for adaptive shapes
- manifest.json: Complete PWA manifest with theme color #4af0c0,
  standalone display mode, shortcuts, and icon definitions
- index.html: Service worker registration, Apple PWA meta tags,
  theme colors, and MS application config

The Nexus now works offline after first visit and can be installed
to home screen on mobile and desktop devices.

Fixes #14

manifest.json

@@ -1,20 +1,61 @@
 {
-  "name": "Timmy's Nexus",
+  "name": "The Nexus",
   "short_name": "Nexus",
+  "description": "Timmy's sovereign 3D world — a Three.js environment serving as the central hub for all portals",
   "start_url": "/",
-  "display": "fullscreen",
-  "background_color": "#050510",
-  "theme_color": "#050510",
+  "display": "standalone",
+  "display_override": ["fullscreen", "minimal-ui"],
+  "orientation": "any",
+  "background_color": "#0a1628",
+  "theme_color": "#4af0c0",
+  "categories": ["entertainment", "games"],
+  "lang": "en",
+  "dir": "ltr",
+  "scope": "/",
   "icons": [
     {
-      "src": "icons/t-logo-192.png",
-      "sizes": "192x192",
-      "type": "image/png"
+      "src": "icons/nexus-icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
     },
     {
-      "src": "icons/t-logo-512.png",
-      "sizes": "512x512",
-      "type": "image/png"
+      "src": "icons/nexus-maskable.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "maskable"
     }
-  ]
+  ],
+  "screenshots": [
+    {
+      "src": "screenshots/nexus-wide.png",
+      "sizes": "1280x720",
+      "type": "image/png",
+      "form_factor": "wide",
+      "label": "The Nexus 3D environment"
+    },
+    {
+      "src": "screenshots/nexus-narrow.png",
+      "sizes": "750x1334",
+      "type": "image/png",
+      "form_factor": "narrow",
+      "label": "The Nexus on mobile"
+    }
+  ],
+  "shortcuts": [
+    {
+      "name": "Enter Nexus",
+      "short_name": "Enter",
+      "description": "Jump directly into the Nexus world",
+      "url": "/?action=enter",
+      "icons": [
+        {
+          "src": "icons/nexus-icon.svg",
+          "sizes": "any"
+        }
+      ]
+    }
+  ],
+  "related_applications": [],
+  "prefer_related_applications": false
 }