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/audio/sounds.ts

@@ -1,5 +1,5 @@
-import { getSettings } from "../storage/db";
 import { getDB } from "../app";
+import { getSettings } from "../storage/db";
 
 let audioCtx: AudioContext | null = null;
 
@@ -83,12 +83,12 @@ export async function playRewardSound(): Promise<void> {
 		const noteDuration = 0.15;
 		const now = ctx.currentTime;
 
-		for (let i = 0; i < notes.length; i++) {
+		for (const [i, freq] of notes.entries()) {
 			const osc = ctx.createOscillator();
 			const gain = ctx.createGain();
 
 			osc.type = "sine";
-			osc.frequency.value = notes[i]!;
+			osc.frequency.value = freq;
 			gain.gain.value = 0.12;
 
 			osc.connect(gain);