fix(web): use FileReader to decode beacon Blob in events test

jsdom's Blob doesn't ship .text() (only the polyfill via FileReader
or arrayBuffer is reliable). The pagehide test failed with
"TypeError: blob.text is not a function" at events.svelte.test.ts:183.

Swap blob.text() for new FileReader().readAsText() wrapped in a
Promise.

web/src/lib/player/events.svelte.test.ts

@@ -179,8 +179,14 @@ describe('useEventsDispatcher', () => {
 
     expect(beacon).toHaveBeenCalled();
     expect(beacon.mock.calls[0][0]).toBe('/api/events');
+    // jsdom's Blob doesn't ship .text(); FileReader does.
     const blob = beacon.mock.calls[0][1] as Blob;
-    const text = await blob.text();
+    const text = await new Promise<string>((resolve, reject) => {
+      const reader = new FileReader();
+      reader.onloadend = () => resolve(reader.result as string);
+      reader.onerror = () => reject(reader.error);
+      reader.readAsText(blob);
+    });
     const payload = JSON.parse(text) as { type: string; duration_played_ms: number };
     expect(payload.type).toBe('play_ended');
     expect(payload.duration_played_ms).toBe(73000);