feat(website): implement Clean URLs and split Caddy configurations

- Removed all .html file extension suffixes from website and invite links (impressum, datenschutz, join).
- Updated app.js dynamic localizer and path switching to target Clean URLs ('./', '../', etc.).
- Refactored Caddyfile.example into simple (lightweight try_files) and advanced (hardened production) options.
- Split Caddy configs into Caddyfile (Simple) and Caddyfile (Advanced) tabs on the landing page.

website/join.html

@@ -11,7 +11,7 @@
 
     <!-- Open Graph / Facebook -->
     <meta property="og:type" content="website">
-    <meta property="og:url" content="https://sync.koalastuff.net/join.html">
+    <meta property="og:url" content="https://sync.koalastuff.net/join">
     <meta property="og:title" content="Join Room | KoalaSync">
     <meta property="og:description" content="You've been invited to watch videos together in perfect sync with your friends on KoalaSync.">
     <meta property="og:image" content="https://sync.koalastuff.net/assets/PlatformJuggler_New.webp">
@@ -38,13 +38,13 @@
 
     <nav>
         <div class="container nav-content">
-            <a href="index.html" class="logo-area" style="text-decoration: none;">
+            <a href="./" class="logo-area" style="text-decoration: none;">
                 <img src="assets/NewLogoIcon.webp" alt="KoalaSync Logo" width="40" height="40">
                 <span>KoalaSync</span>
             </a>
             <button class="hamburger" aria-label="Menu" aria-expanded="false">☰</button>
             <div class="nav-links">
-                <a href="index.html" style="display: inline-flex; align-items: center; gap: 6px;">
+                <a href="./" style="display: inline-flex; align-items: center; gap: 6px;">
                     <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true" style="width: 16px; height: 16px; display: block;"><path d="m3 9 9-7 9 7v11a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"></path><polyline points="9 22 9 12 15 12 15 22"></polyline></svg>
                     <span lang="de">Startseite</span><span lang="en">Home</span>
                 </a>
@@ -107,8 +107,8 @@
             <p lang="en" style="font-size: 0.8rem; margin-top: 0.5rem;">No data is stored on our servers. Pure RAM-based relay.</p>
             <p lang="de" style="font-size: 0.8rem; margin-top: 0.5rem;">Keine Daten werden auf unseren Servern gespeichert. Reines RAM-basiertes Relay.</p>
             <div style="margin-top: 1.5rem; font-size: 0.8rem; display: flex; justify-content: center; align-items: center; gap: 1.5rem; flex-wrap: wrap;">
-                <a href="impressum.html" style="color: var(--text-muted); text-decoration: none;"><span lang="en">Legal Notice</span><span lang="de">Impressum</span></a>
-                <a href="datenschutz.html" style="color: var(--text-muted); text-decoration: none;"><span lang="en">Privacy Policy</span><span lang="de">Datenschutz</span></a>
+                <a href="impressum" style="color: var(--text-muted); text-decoration: none;"><span lang="en">Legal Notice</span><span lang="de">Impressum</span></a>
+                <a href="datenschutz" style="color: var(--text-muted); text-decoration: none;"><span lang="en">Privacy Policy</span><span lang="de">Datenschutz</span></a>
                 <a href="https://mastodon.social/@koalastuff" rel="me" target="_blank" style="color: var(--text-muted); text-decoration: none; display: inline-flex; align-items: center; gap: 4px;">
                     <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="currentColor" width="14" height="14" aria-hidden="true" style="display: block;"><path d="M23.268 5.313c-.35-2.578-2.617-4.61-5.304-5.004C17.51.242 15.792 0 11.813 0h-.03c-3.98 0-4.835.242-5.288.309C3.882.692 1.496 2.518.917 5.127.64 6.412.61 7.837.661 9.143c.074 1.874.088 3.745.26 5.611.118 1.24.325 2.47.62 3.68.55 2.237 2.777 4.098 4.96 4.857 2.336.792 4.849.923 7.256.38.265-.061.527-.132.786-.213.585-.184 1.27-.39 1.774-.753a.057.057 0 0 0 .023-.043v-1.809a.052.052 0 0 0-.02-.041.053.053 0 0 0-.046-.01 20.282 20.282 0 0 1-4.709.545c-2.73 0-3.463-1.284-3.674-1.818a5.593 5.593 0 0 1-.319-1.433.053.053 0 0 1 .066-.054c1.517.363 3.072.546 4.632.546.376 0 .75 0 1.125-.01 1.57-.044 3.224-.124 4.768-.422.038-.008.077-.015.11-.024 2.435-.464 4.753-1.92 4.989-5.604.008-.145.03-1.52.03-1.67.002-.512.167-3.63-.024-5.545zm-3.748 9.195h-2.561V8.29c0-1.309-.55-1.976-1.67-1.976-1.23 0-1.846.79-1.846 2.35v3.403h-2.546V8.663c0-1.56-.617-2.35-1.848-2.35-1.112 0-1.668.668-1.67 1.977v6.218H4.822V8.102c0-1.31.337-2.35 1.011-3.12.696-.77 1.608-1.164 2.74-1.164 1.311 0 2.302.5 2.962 1.498l.638 1.06.638-1.06c.66-.999 1.65-1.498 2.96-1.498 1.13 0 2.043.395 2.74 1.164.675.77 1.012 1.81 1.012 3.12z"/></svg>
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