Add image/attachment support in preview mode

- Fetch attachments via WebDAV using Tauri HTTP plugin (bypasses CORS)
- Parse markdown for image references and convert to base64 data URLs
- In-memory cache to avoid re-fetching images
- Loading indicator while images load
- Register tauri-plugin-http in Rust builder
- Add HTTP permissions in capabilities

package-lock.json

@@ -10,6 +10,7 @@
       "dependencies": {
         "@tauri-apps/api": "^2",
         "@tauri-apps/plugin-dialog": "^2.6.0",
+        "@tauri-apps/plugin-http": "^2.5.7",
         "@tauri-apps/plugin-opener": "^2",
         "@tiptap/extension-strike": "^2.27.2",
         "@tiptap/extension-underline": "^2.27.2",
@@ -1499,6 +1500,15 @@
         "@tauri-apps/api": "^2.8.0"
       }
     },
+    "node_modules/@tauri-apps/plugin-http": {
+      "version": "2.5.7",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-http/-/plugin-http-2.5.7.tgz",
+      "integrity": "sha512-+F2lEH/c9b0zSsOXKq+5hZNcd9F4IIKCK1T17RqMwpCmVnx2aoqY8yIBccCd25HTYUb3j6NPVbRax/m00hKG8A==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.10.1"
+      }
+    },
     "node_modules/@tauri-apps/plugin-opener": {
       "version": "2.5.3",
       "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-opener/-/plugin-opener-2.5.3.tgz",