fix(routes): serve pages with clean URLs (no .html extension)

Add server-side routes for /profile, /admin, and /shared that
serve their respective HTML pages directly (same pattern already
used for /login). Updated all frontend references to use clean
URLs instead of .html extensions.

Fixes #99

static/profile.html

@@ -143,7 +143,7 @@ body{background:#f5f7fa;color:#1e293b;min-height:100vh;display:flex;flex-directi
     <div class="err-icon"><i class="fas fa-lock"></i></div>
     <h2>Not Authenticated</h2>
     <p>Please sign in to view your profile.</p>
-    <a href="/login.html"><i class="fas fa-sign-in-alt"></i> Sign in</a>
+    <a href="/login"><i class="fas fa-sign-in-alt"></i> Sign in</a>
   </div>
 
   <div id="main-content" style="display:none">