Add GamepadAPI, WebGL readPixels noise, auto-prune, import/export, badge

New fingerprint vectors:
- Gamepad API: getGamepads() returns empty (prevents controller fingerprinting)
- WebGL readPixels: seeded pixel noise on framebuffer reads

New features:
- Auto-prune: configurable automatic removal of inactive containers
- Import/export: backup and restore all settings from options page
- Toolbar badge: shows active container count
- CHANGELOG.md: version history

Bumped to v0.5.0 with 20 spoofed fingerprint vectors.

manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 2,
   "name": "ContainSite",
-  "version": "0.4.1",
+  "version": "0.5.0",
   "description": "Per-container fingerprint isolation — each container gets its own device identity",
   "permissions": [
     "contextualIdentities",