refactor(dashboard): change SpawnModal from overlay modal to dropdown

Position the spawn modal directly under the 'New Agent' button without a
blur overlay. Uses click-outside dismissal and absolute positioning.
Reduces visual disruption for quick agent spawning.

dashboard/components/App.js

@@ -471,13 +471,21 @@ export function App() {
               `;
             })()}
           </div>
-          <button
-            disabled=${!zellijAvailable}
-            class="rounded-lg border border-active/40 bg-active/12 px-3 py-2 text-sm font-medium text-active transition-colors hover:bg-active/20 ${!zellijAvailable ? 'opacity-50 cursor-not-allowed' : ''}"
-            onClick=${() => setSpawnModalOpen(true)}
-          >
-            + New Agent
-          </button>
+          <div class="relative">
+            <button
+              disabled=${!zellijAvailable}
+              class="rounded-lg border border-active/40 bg-active/12 px-3 py-2 text-sm font-medium text-active transition-colors hover:bg-active/20 ${!zellijAvailable ? 'opacity-50 cursor-not-allowed' : ''}"
+              onClick=${() => setSpawnModalOpen(true)}
+            >
+              + New Agent
+            </button>
+            <${SpawnModal}
+              isOpen=${spawnModalOpen}
+              onClose=${() => setSpawnModalOpen(false)}
+              onSpawn=${handleSpawnResult}
+              currentProject=${selectedProject}
+            />
+          </div>
         </div>
       </header>
 
@@ -589,12 +597,5 @@ export function App() {
     />
 
     <${ToastContainer} />
-
-    <${SpawnModal}
-      isOpen=${spawnModalOpen}
-      onClose=${() => setSpawnModalOpen(false)}
-      onSpawn=${handleSpawnResult}
-      currentProject=${selectedProject}
-    />
   `;
 }