feat: add character avatars with upload and zoom functionality

- Add avatar upload with file picker dialog
- Display avatars in header, chat messages, and settings
- Implement clickable avatars with full-screen zoom modal
- Enable asset protocol for local file access in Tauri config
- Add tauri-plugin-dialog for native file selection
- Store avatars in ~/.config/claudia/avatars/
- Support PNG, JPG, JPEG, and WEBP formats
- Modal closes on overlay click or ESC key

src-tauri/tauri.conf.json

@@ -24,7 +24,11 @@
       }
     ],
     "security": {
-      "csp": null
+      "csp": null,
+      "assetProtocol": {
+        "enable": true,
+        "scope": ["$HOME/.config/claudia/**"]
+      }
     }
   },
   "bundle": {