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/reader.templ

@@ -17,6 +17,7 @@ func readerInitExpr(metadata ReaderMetadata) string {
 		"readingDirection": metadata.ReadingDirection,
 		"mangaType":        metadata.MangaType,
 		"treatAsComic":     metadata.TreatAsComic,
+		"keepScreenAwake":  metadata.KeepScreenAwake,
 	}
 	jsonBytes, _ := json.Marshal(config)
 	return fmt.Sprintf("initReader(%s)", string(jsonBytes))