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
Follow-up to ce9971b — the logo was only fixed in index.html.
Now profile.html, admin.html, and shared.html also wrap their
logo elements in <a href='/'> links.
Fixes#97
Dionisio