fix(mixin): access platformService as property, not function, in PlatformServiceMixin

- Fix all computed properties to use platformService as a property
- Add descriptive @ts-expect-error comments for dynamic property access
- Resolves 'this.platformService is not a function' runtime error
- Lint clean

src/components/DataExportSection.vue

@@ -100,6 +100,13 @@ export default class DataExportSection extends Vue {
    */
   notify = createNotifyHelpers(this.$notify);
 
+  /**
+   * NOTE: PlatformServiceMixin provides both concise helpers (e.g. $contacts, capabilities)
+   * and the full platformService instance for advanced/native features (e.g. writeAndShareFile).
+   * Always use 'this.platformService' as a property (never as a function).
+   */
+  declare readonly platformService: import("@/services/PlatformService").PlatformService;
+
   /**
    * Computed property to check if we're on web platform
    */