feat(player): persist volume across sessions, default to 100%

Store the playback volume in app settings (config.json) whenever it
changes and re-apply the previous session's level at boot, instead of
always starting at the old 70% fallback.

- AppSettings gains volume (default 1 = 100%); both default-settings
  copies and the volume signal default are raised from 0.7 to 1.
- doSetVolume persists via the app store (mirrors playbackSpeed).
- The boot sync awaits the app store's async config load (new
  whenReady()) so a persisted level is applied even when settings load
  finishes after useAudio mounts.
- tests/volume-persistence.test.ts: default, clamp, and cross-session
  reuse (fresh module instance simulates the next launch).

tests/volume-persistence.test.ts

@@ -0,0 +1,85 @@
+/**
+ * volume-persistence.test.ts — "store and reuse the previous session's
+ * audio level" feature.
+ *
+ * useAudio's volume starts at 100% (default, before any user change);
+ * setVolume() persists the new level to app settings (config.json); and
+ * at boot the volume signal re-syncs from the persisted settings — so the
+ * next session resumes at the previous level.
+ *
+ * Same worker-leak defenses as restore-session.test.ts: other test files
+ * mock.module("../src/hooks/useAudio") and bun reuses workers, so the real
+ * module is imported via a query-suffixed specifier (distinct module
+ * identity, loads from disk). A fresh module instance simulates the next
+ * launch: its refCount starts at 0, so its boot sync re-reads settings.
+ * The boot sync awaits the app store's async config load (whenReady), so
+ * tests await it too and flush microtasks — no wall-clock sleeps.
+ */
+import { test, expect } from "bun:test";
+import { mkdtempSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+// ── Sandbox BEFORE any app module evaluates ───────────────────────────────
+const CONFIG = mkdtempSync(join(tmpdir(), "podtui-volume-"));
+process.env.XDG_CONFIG_HOME = CONFIG;
+process.env.XDG_DATA_HOME = mkdtempSync(join(tmpdir(), "podtui-volume-data-"));
+process.env.PODTUI_AUDIO_BACKEND = "none";
+
+// ── Real modules ──────────────────────────────────────────────────────────
+// @ts-expect-error — bun-only query suffix: distinct module identity that
+// loads the real file instead of a leaked mock.module from another test file.
+const { useAudio } = await import("../src/hooks/useAudio?restore-test");
+const { useAppStore } = await import("../src/stores/app");
+
+/** Flush the boot sync's promise chain (whenReady.then(...)) — microtasks
+ *  only, no timers. */
+async function flushMicrotasks(): Promise<void> {
+	for (let i = 0; i < 5; i++) await Promise.resolve();
+}
+
+test("volume defaults to 100% and is persisted and reused across sessions", async () => {
+	const appStore = useAppStore();
+	await appStore.whenReady(); // empty sandbox config → defaults
+
+	// Boot 1: no persisted volume — the default is 100% (not the old 70%).
+	const audio = useAudio();
+	await flushMicrotasks();
+	expect(audio.volume()).toBe(1);
+	expect(appStore.state().settings.volume).toBe(1);
+
+	// User change: signal updates and the level lands in app settings.
+	await audio.setVolume(0.35);
+	expect(audio.volume()).toBe(0.35);
+	expect(appStore.state().settings.volume).toBe(0.35);
+
+	// Boot 2 (fresh module instance — refCount starts at 0, so the boot
+	// sync re-reads settings): the previous session's level is restored.
+	// @ts-expect-error — same bun-only query-suffix mechanism as above.
+	const { useAudio: useAudioNext } = await import("../src/hooks/useAudio?restore-test-vol");
+	const nextAudio = useAudioNext();
+	await flushMicrotasks();
+	expect(nextAudio.volume()).toBe(0.35);
+});
+
+test("setVolume clamps to the 0–1 range before persisting", async () => {
+	const audio = useAudio();
+	await audio.setVolume(1.7);
+	expect(audio.volume()).toBe(1);
+	expect(useAppStore().state().settings.volume).toBe(1);
+
+	await audio.setVolume(-0.3);
+	expect(audio.volume()).toBe(0);
+	expect(useAppStore().state().settings.volume).toBe(0);
+});
+
+test("a persisted volume wins over the default at boot", async () => {
+	// Persist a non-default level, then simulate a fresh launch that has no
+	// prior signal state (new module instance).
+	useAppStore().updateSettings({ volume: 0.6 });
+	// @ts-expect-error — same bun-only query-suffix mechanism as above.
+	const { useAudio: useAudioNext } = await import("../src/hooks/useAudio?restore-test-vol2");
+	const nextAudio = useAudioNext();
+	await flushMicrotasks();
+	expect(nextAudio.volume()).toBe(0.6);
+});