feat(audio-player): settings popover + autoplay-on-open (plan §4)

Header gear (beside the keyboard icon) opens a 'Player settings'
popover — one option row today, structured for more: 'Autoplay on
open', a role=switch toggle persisted to localStorage audio_autoplay
('1'/'0', default off) via the normalizeAutoplay one-source helper.

maybeAutoplay() runs strictly AFTER restorePosition resolves: the
?loc=audio:<seconds> deep link keeps winning on position; the setting
only decides whether playback starts. A browser autoplay-policy
rejection (or any play() failure) is swallowed silently — the user
just taps play; the unhandled-rejection trap in init would otherwise
shout 'REJ:' into the title. settingsOpen joins closeTopSurface, so
Esc closes the panel with the other surfaces.

Unit: normalizeAutoplay tests. Live: 13 Playwright checks — default
off stays paused, toggle + persistence, reload autoplays with no
gesture, deep link lands at 600s global while autoplay starts, off
stays paused, and a stubbed NotAllowedError proves the silent
swallow; probe user created and deleted.

web/src/audio.test.ts

@@ -1,6 +1,13 @@
 import { describe, expect, it } from "vitest";
 
-import { SPEEDS, normalizeSpeed, stepSpeed, SKIP_SECONDS, cycleSkip } from "./audio";
+import {
+  SPEEDS,
+  normalizeSpeed,
+  stepSpeed,
+  SKIP_SECONDS,
+  cycleSkip,
+  normalizeAutoplay,
+} from "./audio";
 
 describe("SPEEDS", () => {
   it("is the agreed list from the player plan (0.5 through 3)", () => {
@@ -71,3 +78,18 @@ describe("SKIP_SECONDS + cycleSkip", () => {
     expect(cycleSkip(7, -1)).toBe(5);
   });
 });
+
+describe("normalizeAutoplay", () => {
+  it("is on only for explicitly-on values", () => {
+    expect(normalizeAutoplay("1")).toBe(true);
+    expect(normalizeAutoplay("true")).toBe(true);
+  });
+
+  it("defaults to off for null, off, and garbage", () => {
+    expect(normalizeAutoplay(null)).toBe(false);
+    expect(normalizeAutoplay("0")).toBe(false);
+    expect(normalizeAutoplay("")).toBe(false);
+    expect(normalizeAutoplay("on")).toBe(false);
+    expect(normalizeAutoplay("2")).toBe(false);
+  });
+});