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/js/shared.js

@@ -15,7 +15,7 @@ function checkAuthentication() {
     
     if (!token || !tokenExpiry || new Date(tokenExpiry) < new Date()) {
         // No token or expired token
-        window.location.href = '/login.html';
+        window.location.href = '/login';
         return;
     }