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

index.html

@@ -191,6 +191,10 @@
 			</div>
 		</div>
 
+		<button id="mute-btn" class="mute-btn" aria-label="Audio stumm schalten" title="Audio">
+			<span class="mute-btn__icon" id="mute-icon">&#x1F50A;</span>
+		</button>
+
 		<script type="module" src="/src/main.ts"></script>
 	</body>
 </html>