feat: add session import/export functionality

- Add JSON export with full session metadata for backup/restore
- Add Markdown export with formatted conversation history
- Add import capability for previously exported sessions
- Add tauri-plugin-fs for file system operations
- Add export dropdown menu and import button to session history panel

src-tauri/capabilities/default.json

@@ -16,6 +16,9 @@
     "notification:allow-notify",
     "clipboard-manager:default",
     "clipboard-manager:allow-read-image",
-    "core:tray:default"
+    "core:tray:default",
+    "fs:default",
+    "fs:allow-read-text-file",
+    "fs:allow-write-text-file"
   ]
 }