fix(spawn): handle empty or missing projects directory

- Add empty-state message in SpawnModal when no projects found
- Spawn button stays disabled when projects list is empty
- Server already handled OSError/missing dir gracefully (returns [])
- Add tests: missing directory, only-hidden-dirs, empty API responses

Closes: bd-3c7

dashboard/components/SpawnModal.js

@@ -86,7 +86,7 @@ export function SpawnModal({ isOpen, onClose, onSpawn, currentProject }) {
       const data = await response.json();
 
       if (data.ok) {
-        onSpawn({ success: true, project, agentType });
+        onSpawn({ success: true, project, agentType, spawnId: data.spawn_id });
         handleClose();
       } else {
         setError(data.error || 'Spawn failed');
@@ -139,6 +139,10 @@ export function SpawnModal({ isOpen, onClose, onSpawn, currentProject }) {
                   <span class="working-dots"><span>.</span><span>.</span><span>.</span></span>
                   Loading projects
                 </div>
+              ` : projects.length === 0 ? html`
+                <div class="rounded-xl border border-selection/75 bg-bg/70 px-3 py-2 text-sm text-dim">
+                  No projects found in ~/projects/
+                </div>
               ` : html`
                 <select
                   value=${selectedProject}