feat: delete traffic targets and LLM task conversations from UI

Add delete controls to the Traffic and LLM Records pages so users can
purge recorded data without hand-editing the data volume.

Traffic page:
- Host target picker (Popover with checkboxes + select-all toggle)
- Exact-match batch delete for multiple hosts (DELETE /api/traffic/hosts)
- Substring delete for the host-filter input (DELETE /api/traffic?host=)
- Orphaned content-addressed blob garbage collection after deletes
- Distinct-hosts endpoint (GET /api/traffic/hosts) for the picker

LLM Records page:
- Task picker dropdown (distinct task_id + record counts)
- Per-task delete (DELETE /api/llm/records?task=)

Other:
- Add /dist/ to .gitignore (build artifact)
- Extend traffic_test.go with DeleteHost, DeleteHostsExact, Hosts tests
- Fix Popover render bug (replace ScrollArea that broke height calc)
- Remove fade/zoom animation on the host picker Popover

.gitignore

@@ -37,6 +37,7 @@ Thumbs.db
 # 内嵌前端的单二进制产物
 /artex
 /server/webui/dist/
+/dist/
 
 # Docker 部署生成的本地环境（含密码/密钥，不提交）
 .env