feat: add language switcher between English and German

- Add language toggle button in header (EN/DE)
- Link directly to the other language version
- Style consistent with existing header buttons
- Accessible with proper aria-labels

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/index.de.html

@@ -20,7 +20,10 @@
 					<img src="./bowl.png" width="40" alt="CODE CRISPIES Logo" />
 					<h1>CODE<span>CRISPIES</span></h1>
 				</div>
-				<button id="help-btn" class="help-toggle" aria-label="Hilfe">?</button>
+				<div class="header-actions">
+					<a href="./index.html" class="lang-switch" aria-label="Switch language: English">EN</a>
+					<button id="help-btn" class="help-toggle" aria-label="Hilfe">?</button>
+				</div>
 			</header>
 
 			<!-- Hauptlayout -->