fix(journal): decouple broker from session

The broker connection now persists across session changes. Joining or switching a session swaps subscriptions instead of reconnecting, and a lobby lets sessions be listed and created before joining one.

Fixes session switching being a no-op (the store only updated local state) and session creation being unreachable before connecting. Also removes the dead ConnectionBar and the no-op hydrateFromServer/setSessionId store calls.

src/components/journal/JournalHeader.tsx

@@ -1,8 +1,8 @@
 /**
- * JournalHeader — status dot, player name, session dropdown, disconnect & close
+ * JournalHeader — status dot, player name, session dropdown, leave & close
  */
 import { Component, Show } from "solid-js";
-import { useJournalStream, disconnectStream } from "../stores/journalStream";
+import { useJournalStream, leaveSession } from "../stores/journalStream";
 import { SessionDropdown } from "./SessionDropdown";
 
 interface JournalHeaderProps {
@@ -13,7 +13,7 @@ interface JournalHeaderProps {
 export const JournalHeader: Component<JournalHeaderProps> = (props) => {
   const stream = useJournalStream();
 
-  const handleDisconnect = () => disconnectStream();
+  const handleLeave = () => leaveSession();
 
   return (
     <div class="flex items-center justify-between px-3 py-2 border-b border-gray-200 shrink-0">
@@ -32,7 +32,9 @@ export const JournalHeader: Component<JournalHeaderProps> = (props) => {
         <Show
           when={stream.connected && stream.sessionId}
           fallback={
-            <h2 class="text-sm font-semibold text-gray-700 truncate">未连接</h2>
+            <h2 class="text-sm font-semibold text-gray-700 truncate">
+              {stream.connected ? "选择会话" : "未连接"}
+            </h2>
           }
         >
           <div class="min-w-0">
@@ -58,13 +60,13 @@ export const JournalHeader: Component<JournalHeaderProps> = (props) => {
         </Show>
       </div>
       <div class="flex items-center gap-1 shrink-0">
-        <Show when={stream.connected}>
+        <Show when={stream.sessionId}>
           <button
-            onClick={handleDisconnect}
-            class="text-xs text-gray-400 hover:text-red-500 px-1"
-            title="断开连接"
+            onClick={handleLeave}
+            class="text-xs text-gray-400 hover:text-blue-500 px-1"
+            title="返回会话列表"
           >
-            ⏻
+            ←
           </button>
         </Show>
         <button