feat(04-01): add mute button with CSS, wire audio toggle in main.ts

- Add mute-btn element to index.html outside all screen sections
- Add .mute-btn CSS styles (fixed position, bottom-right, circular)
- Wire mute button click handler to toggle audioEnabled in Settings
- Initialize AudioContext on first user interaction (autoplay policy)
- Fix lint issues: import ordering, non-null assertion

src/main.ts

@@ -1,5 +1,7 @@
 import "./styles/main.css";
-import { initApp } from "./app";
+import { getDB, initApp } from "./app";
+import { initAudioOnInteraction } from "./audio/sounds";
+import { getSettings, saveSettings } from "./storage/db";
 
 window.addEventListener("error", (event) => {
 	console.error("Global error:", event.error);
@@ -13,4 +15,51 @@ window.addEventListener("unhandledrejection", (event) => {
 	if (overlay) overlay.style.display = "flex";
 });
 
-document.addEventListener("DOMContentLoaded", initApp);
+document.addEventListener("DOMContentLoaded", async () => {
+	await initApp();
+
+	const db = getDB();
+
+	// Wire mute button
+	const muteBtn = document.getElementById("mute-btn");
+	const muteIcon = document.getElementById("mute-icon");
+	if (muteBtn && muteIcon) {
+		// Set initial state from settings
+		const settings = await getSettings(db);
+		const audioEnabled = settings?.audioEnabled !== false;
+		muteIcon.textContent = audioEnabled ? "\u{1F50A}" : "\u{1F507}";
+		if (!audioEnabled) {
+			muteBtn.classList.add("mute-btn--muted");
+		}
+
+		muteBtn.onclick = async () => {
+			// Initialize AudioContext on first click (autoplay policy)
+			initAudioOnInteraction();
+
+			const current = await getSettings(db);
+			const nowEnabled = !(current?.audioEnabled !== false);
+			const updated = current ?? {
+				id: 1 as const,
+				audioEnabled: false,
+				speechEnabled: false,
+				apiKey: "",
+				apiCallsToday: { text: 0, image: 0 },
+				lastApiCallDate: "",
+			};
+			updated.audioEnabled = nowEnabled;
+			await saveSettings(db, updated);
+
+			muteIcon.textContent = nowEnabled ? "\u{1F50A}" : "\u{1F507}";
+			muteBtn.classList.toggle("mute-btn--muted", !nowEnabled);
+		};
+	}
+
+	// Initialize AudioContext on first user interaction (autoplay policy D-02)
+	const initAudioOnce = () => {
+		initAudioOnInteraction();
+		document.removeEventListener("click", initAudioOnce);
+		document.removeEventListener("keydown", initAudioOnce);
+	};
+	document.addEventListener("click", initAudioOnce);
+	document.addEventListener("keydown", initAudioOnce);
+});