Project folders for the agent, Local, the menu under its button

- Settings… lists the folders the agent may work in, with the system's
  folder dialog, remove, max mode and terminal, all through
  'lembas remote … --json' (CLI 1.2.0). Turning the agent on with no
  folder opens it; the title bar says 'agent: no folders' instead of green.
- The content webview's User-Agent carries LLeMbasDesktop/<v> (host=…),
  so the server can show this computer's device as Local.
- The title bar menu pops up at the button (popup_menu_at): Wayland gave
  the pointer-less popup the middle of the window.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ui/titlebar.css

@@ -64,7 +64,8 @@ button:focus-visible { outline: 2px solid var(--accent); outline-offset: -2px; }
 .bar__agent[data-state="elsewhere"] .dot { background: var(--leaf); }
 .bar__agent[data-state="starting"] .dot { background: var(--accent); }
 .bar__agent[data-state="failed"] .dot,
-.bar__agent[data-state="not_linked"] .dot { background: var(--warning); }
+.bar__agent[data-state="not_linked"] .dot,
+.bar__agent[data-state="no_folders"] .dot { background: var(--warning); }
 
 .bar__update {
   padding: 0 var(--sp-3);