feat(web): hold a Screen Wake Lock while reading or listening

New wake-lock.ts engine (sanctioned island, dependency-injected for
tests): feature-detected navigator.wakeLock, silent no-op when
unsupported or denied, and the spec-mandated force-release-on-hide →
re-acquire-on-visible loop handled via the sentinel release event.
The player acquires on play and releases on pause (sleep-timer fades
ride the same path); the reader holds the lock for the session. Both
gate on the keep_screen_awake preference — data-keep-awake on the
player root, keepScreenAwake in readerInitExpr.

Hidden-tab playback intentionally does not keep the screen awake:
the spec forbids screen wake locks for hidden documents.

templates/audio_reader.templ

@@ -40,6 +40,7 @@ templ AudioReader(user User, metadata ReaderMetadata) {
 			data-title={ metadata.Title }
 			data-cover={ metadata.CoverImagePath }
 			data-file-url={ metadata.FileURL }
+			data-keep-awake={ metadata.KeepScreenAwake }
 			data-json={ fmt.Sprintf(`{"chapterEndpoint":"%s","progressEndpoint":"%s"}`,
 				"/api/media-items/"+metadata.MediaItemID+"/chapters",
 				"/api/media-items/"+metadata.MediaItemID+"/progress") }