fix: improve file sharing on Android using app-private storage

- Replace direct file writing with app-private storage + share dialog
- Add Share plugin for cross-platform file sharing
- Update file paths configuration for Android
- Fix permission issues by using Directory.Data instead of Documents
- Simplify file export flow in DataExportSection

src/components/DataExportSection.vue

@@ -145,7 +145,7 @@ export default class DataExportSection extends Vue {
       } else if (this.platformCapabilities.hasFileSystem) {
         // Native platform: Write to app directory
         const content = await blob.text();
-        await this.platformService.writeFile(fileName, content);
+        await this.platformService.writeAndShareFile(fileName, content);
       }
 
       this.$notify(