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

Add full Progressive Web App support to The Nexus:
- manifest.json: Web app manifest with Nexus branding, icons, and PWA config
- 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/: SVG icons (standard + maskable) with crystalline Nexus design
- index.html: Add manifest link, theme colors, Apple PWA meta tags,
  and service worker registration script

Features:
- Works offline after first visit
- Installable to home screen on mobile and desktop
- Graceful degradation when offline
- Auto-refresh when connection restored

Fixes #14

manifest.json

@@ -0,0 +1,61 @@
+{
+  "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": "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/nexus-icon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any"
+    },
+    {
+      "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
+}