Move web UI from /ui/ to /web/ path prefix

The web UI is now mounted at /web/ (previously /ui/) — cmd/server/main.go
wraps web.Server.Handler with http.StripPrefix("/web", ...), so
internal/web's own routes stay unprefixed (/, /login, /logout,
/shares/..., /static/...) and only the outer mux adds the prefix. All
templates, redirects, and cookie paths updated accordingly. The root '/'
route reverts to the original unauthenticated welcome page (linking to
/web/), and /cal/, /card/, /files/ are unaffected.

Also gitignore the bin/ build output directory.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/web/session.go

@@ -26,13 +26,13 @@ func (s *Server) requireLogin(next http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		cookie, err := r.Cookie(sessionCookieName)
 		if err != nil {
-			http.Redirect(w, r, "/ui/login", http.StatusSeeOther)
+			http.Redirect(w, r, "/web/login", http.StatusSeeOther)
 			return
 		}
 		username, err := s.dbase.SessionUser(cookie.Value)
 		if err != nil {
 			s.clearSessionCookie(w)
-			http.Redirect(w, r, "/ui/login", http.StatusSeeOther)
+			http.Redirect(w, r, "/web/login", http.StatusSeeOther)
 			return
 		}
 		ctx := context.WithValue(r.Context(), userCtxKey, username)