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/JournalHeader.tsx

@@ -7,6 +7,7 @@ import { SessionDropdown } from "./SessionDropdown";
 
 interface JournalHeaderProps {
   onClose: () => void;
+  onCreateSession: () => void;
 }
 
 export const JournalHeader: Component<JournalHeaderProps> = (props) => {
@@ -51,7 +52,7 @@ export const JournalHeader: Component<JournalHeaderProps> = (props) => {
             </p>
             {/* Clickable subtitle — session dropdown (GM only) */}
             <Show when={stream.myRole === "gm"}>
-              <SessionDropdown />
+              <SessionDropdown onCreate={props.onCreateSession} />
             </Show>
           </div>
         </Show>