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.

web/src/audio.ts

@@ -15,6 +15,7 @@
 import { Alpine } from "./alpine";
 import { renderNoteHtml } from "./markdown";
 import { showToast } from "./toast";
+import { WakeLockManager } from "./wake-lock";
 import {
   MOUSE_GUIDE,
   resolveShortcut,
@@ -288,6 +289,13 @@ Alpine.data("audioReader", () => ({
   // 30s of a countdown. Reset by stop/finish.
   sleepDim: 0,
 
+  // Screen wake lock (keep_screen_awake profile preference): held while
+  // playing so the display doesn't blank/lock mid-listen. The browser
+  // force-releases when the page hides; the manager re-acquires on
+  // visibility regain while audio is still going.
+  wakeLock: new WakeLockManager(),
+  keepAwakeEnabled: false,
+
   // Settings popover (plan §4): header gear → panel. Autoplay-on-open
   // is the first option; the panel is structured for more.
   settingsOpen: false,
@@ -336,6 +344,7 @@ Alpine.data("audioReader", () => ({
     const title = body.getAttribute("data-title") || "";
     const cover = body.getAttribute("data-cover") || "";
     const fileUrl = body.getAttribute("data-file-url") || "";
+    this.keepAwakeEnabled = body.getAttribute("data-keep-awake") === "true";
     this.mediaItemId = mediaId;
     this.title = title;
     this.fileUrl = fileUrl;
@@ -413,6 +422,7 @@ Alpine.data("audioReader", () => ({
       // is awake and asked for audio); stopSleep restores the volume.
       if (this.sleep.mode === "fading") this.stopSleep();
       this.updateMediaSessionState();
+      if (this.keepAwakeEnabled) void this.wakeLock.acquire();
     });
     audio.addEventListener("pause", () => {
       const now = performance.now();
@@ -423,6 +433,7 @@ Alpine.data("audioReader", () => ({
       this.playing = false;
       this.pushProgress(true);
       this.updateMediaSessionState();
+      if (this.keepAwakeEnabled) this.wakeLock.release();
     });
     window.addEventListener("pagehide", () => this.pushProgress(true));
     window.addEventListener("resize", () => this.recomputeTicks());