[Agent Feature] React structure refine and State refine

**What would you like to be added**:
React 架构规整，包括三阶段的精简，State（messages，DB等依赖项）的规整

**Why is this needed**:
see https://github.com/apache/dubbo-admin/discussions/1386

Start with the linked discussion #1386 and map the existing React architecture and state handling for messages and DB dependencies. The issue names no files, tests, or acceptance criteria, so clarify the intended three-stage simplification and agree on what “done” means before coding.

Written by the indexing model from the issue text.