feat(dsh-plugin): better-sidebar 集成、会话关停与归档清理、悬浮窗修复 (#130)

* better-sidebar 集成：observation 视图迁入 better-sidebar 载体，按会话作用域展示 bsk 会话
* 会话关停与归档清理：支持会话 dispose 与归档自动清理
* 悬浮窗修复：悬浮 overlay 拖拽/缩放/PiP 弹出等问题修复
* 修复 Document PiP 空白窗口：载体卸载（关 tab、切会话、插件 HMR）时主动 close() PiP 窗口，避免残留空白窗口

packages/dsh-plugin-browserskill/src/client/sidebar-mode.ts

@@ -0,0 +1,26 @@
+/**
+ * Carrier switch for the observation view: while the better-sidebar
+ * integration fiber is alive (the dsh-better-sidebar plugin provides its
+ * `betterSidebar` service), the tracking view lives in a sidebar tab and the
+ * floating overlay card/capsule hides itself. The flag is a tiny external
+ * store so the overlay can read it through useSyncExternalStore and flip
+ * without a remount when the sidebar plugin (un)loads.
+ */
+
+let active = false;
+const listeners = new Set<() => void>();
+
+export function getSidebarMode(): boolean {
+  return active;
+}
+
+export function subscribeSidebarMode(listener: () => void): () => void {
+  listeners.add(listener);
+  return () => listeners.delete(listener);
+}
+
+export function setSidebarMode(next: boolean): void {
+  if (active === next) return;
+  active = next;
+  for (const listener of [...listeners]) listener();
+}