fix: route CapturesTab exports through the platform layer

The wav/txt/md export handlers called plugin-dialog and plugin-fs
directly, which breaks in the web build. saveFile now returns the
saved path (null on cancel) so callers can toast accurately, and the
cross-window capture listeners only register under Tauri.

web/src/platform/filesystem.ts

@@ -11,6 +11,7 @@ export const webFilesystem: PlatformFilesystem = {
     a.click();
     window.URL.revokeObjectURL(url);
     document.body.removeChild(a);
+    return filename;
   },
 
   async openPath(_path: string) {