feat: implement session creation flow

Refactor `CreateSessionDialog` to be controlled by its parent instead
of using an internal `open` prop. Lift the state up to `JournalPanel`
and pass the creation callback through `JournalHeader` to
`SessionDropdown`.

src/components/journal/SessionDropdown.tsx

@@ -8,13 +8,15 @@ import {
   hydrateFromServer,
   useJournalStream,
 } from "../stores/journalStream";
-import { CreateSessionDialog } from "./CreateSessionDialog";
 
-export const SessionDropdown: Component = () => {
+interface SessionDropdownProps {
+  onCreate: () => void;
+}
+
+export const SessionDropdown: Component<SessionDropdownProps> = (props) => {
   const stream = useJournalStream();
   const manifest = sessions;
   const [dropdownOpen, setDropdownOpen] = createSignal(false);
-  const [showCreateDialog, setShowCreateDialog] = createSignal(false);
   let dropdownRef!: HTMLDivElement;
 
   const handleSessionSelect = (id: string) => {
@@ -75,7 +77,8 @@ export const SessionDropdown: Component = () => {
             onClick={(e) => {
               e.stopPropagation();
               e.preventDefault();
-              setShowCreateDialog(true);
+              setDropdownOpen(false);
+              props.onCreate();
             }}
             class="w-full text-left px-3 py-1 text-xs text-blue-600 hover:bg-gray-100"
           >
@@ -83,10 +86,6 @@ export const SessionDropdown: Component = () => {
           </button>
         </div>
       </Show>
-      <CreateSessionDialog
-        open={showCreateDialog()}
-        onClose={() => setShowCreateDialog(false)}
-      />
     </div>
   );
 };