feat(frontend): polish connection tree and terminal groups

Add explorer-style tree search in the connections view with
match-path expansion, clearer count highlighting, and a refined
sidebar header layout.

Improve terminal tab grouping by keeping new sessions appended
within their server group, highlighting the active group, and
deduplicating broadcast actions to send commands once per server.

.helloagents/plan/202603252310_connections-tree-search-explorer-polish/.status.json

@@ -0,0 +1 @@
+{"status":"in_progress","completed":1,"failed":0,"pending":3,"total":4,"done":1,"percent":25,"current":"在 ConnectionsView 中实现左侧树搜索和资源管理器式头部布局","updated_at":"2026-03-25 23:10:00"}