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

pnpm-lock.yaml

@@ -17,6 +17,9 @@ importers:
       '@tauri-apps/plugin-dialog':
         specifier: ^2
         version: 2.6.0
+      '@tauri-apps/plugin-fs':
+        specifier: ^2.4.5
+        version: 2.4.5
       '@tauri-apps/plugin-notification':
         specifier: ^2
         version: 2.3.3
@@ -744,6 +747,9 @@ packages:
   '@tauri-apps/plugin-dialog@2.6.0':
     resolution: {integrity: sha512-q4Uq3eY87TdcYzXACiYSPhmpBA76shgmQswGkSVio4C82Sz2W4iehe9TnKYwbq7weHiL88Yw19XZm7v28+Micg==}
 
+  '@tauri-apps/plugin-fs@2.4.5':
+    resolution: {integrity: sha512-dVxWWGE6VrOxC7/jlhyE+ON/Cc2REJlM35R3PJX3UvFw2XwYhLGQVAIyrehenDdKjotipjYEVc4YjOl3qq90fA==}
+
   '@tauri-apps/plugin-notification@2.3.3':
     resolution: {integrity: sha512-Zw+ZH18RJb41G4NrfHgIuofJiymusqN+q8fGUIIV7vyCH+5sSn5coqRv/MWB9qETsUs97vmU045q7OyseCV3Qg==}
 
@@ -2300,6 +2306,10 @@ snapshots:
     dependencies:
       '@tauri-apps/api': 2.9.1
 
+  '@tauri-apps/plugin-fs@2.4.5':
+    dependencies:
+      '@tauri-apps/api': 2.9.1
+
   '@tauri-apps/plugin-notification@2.3.3':
     dependencies:
       '@tauri-apps/api': 2.9.1