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/CreateSessionDialog.tsx

@@ -1,11 +1,10 @@
 /**
  * CreateSessionDialog — modal for naming a new session
  */
-import { Component, createSignal, Show } from "solid-js";
+import { Component, createSignal, onMount } from "solid-js";
 import { createSession } from "../stores/journalStream";
 
 interface CreateSessionDialogProps {
-  open: boolean;
   onClose: () => void;
 }
 
@@ -23,56 +22,56 @@ export const CreateSessionDialog: Component<CreateSessionDialogProps> = (
     props.onClose();
   };
 
+  onMount(() => inputRef?.focus());
+
   return (
-    <Show when={props.open}>
+    <div
+      class="absolute inset-0 z-50 flex items-center justify-center bg-black/20"
+      onClick={props.onClose}
+    >
       <div
-        class="absolute inset-0 z-50 flex items-center justify-center bg-black/20"
-        onClick={props.onClose}
+        class="bg-white rounded-lg border border-gray-200 shadow-xl w-[280px] p-4 space-y-3"
+        onClick={(e) => e.stopPropagation()}
       >
-        <div
-          class="bg-white rounded-lg border border-gray-200 shadow-xl w-[280px] p-4 space-y-3"
-          onClick={(e) => e.stopPropagation()}
-        >
-          <div class="flex items-center justify-between">
-            <h3 class="text-sm font-semibold text-gray-800">新建会话</h3>
-            <button
-              onClick={props.onClose}
-              class="text-gray-400 hover:text-gray-600 text-sm"
-            >
-              ✕
-            </button>
-          </div>
-          <p class="text-xs text-gray-500">输入新会话的名称。</p>
-          <input
-            ref={inputRef!}
-            type="text"
-            value={name()}
-            onInput={(e) => setName(e.currentTarget.value)}
-            onKeyDown={(e) => {
-              if (e.key === "Enter") handleCreate();
-              if (e.key === "Escape") props.onClose();
-            }}
-            placeholder="会话名称"
-            class="w-full border border-gray-300 rounded px-2 py-1.5 text-sm"
-            autofocus
-          />
-          <div class="flex justify-end gap-1">
-            <button
-              onClick={props.onClose}
-              class="px-3 py-1 text-xs text-gray-600 hover:text-gray-800"
-            >
-              取消
-            </button>
-            <button
-              onClick={handleCreate}
-              disabled={!name().trim()}
-              class="bg-blue-600 text-white rounded px-3 py-1 text-xs font-medium hover:bg-blue-700 disabled:opacity-50"
-            >
-              创建
-            </button>
-          </div>
+        <div class="flex items-center justify-between">
+          <h3 class="text-sm font-semibold text-gray-800">新建会话</h3>
+          <button
+            onClick={props.onClose}
+            class="text-gray-400 hover:text-gray-600 text-sm"
+          >
+            ✕
+          </button>
+        </div>
+        <p class="text-xs text-gray-500">输入新会话的名称。</p>
+        <input
+          ref={inputRef!}
+          type="text"
+          value={name()}
+          onInput={(e) => setName(e.currentTarget.value)}
+          onKeyDown={(e) => {
+            if (e.key === "Enter") handleCreate();
+            if (e.key === "Escape") props.onClose();
+          }}
+          placeholder="会话名称"
+          class="w-full border border-gray-300 rounded px-2 py-1.5 text-sm"
+          autofocus
+        />
+        <div class="flex justify-end gap-1">
+          <button
+            onClick={props.onClose}
+            class="px-3 py-1 text-xs text-gray-600 hover:text-gray-800"
+          >
+            取消
+          </button>
+          <button
+            onClick={handleCreate}
+            disabled={!name().trim()}
+            class="bg-blue-600 text-white rounded px-3 py-1 text-xs font-medium hover:bg-blue-700 disabled:opacity-50"
+          >
+            创建
+          </button>
         </div>
       </div>
-    </Show>
+    </div>
   );
 };