Document frontend component structure

- Add docs/component-structure.md covering the current React component hierarchy, page routes, public components, API/Auth/Utils boundaries, and data flow.

- Document the internal responsibilities and split risks for large pages including ReportEditor and TemplateManage.

- Link the new component structure guide from README, docs index, AGENTS file tree, and progress notes.

AGENTS.md

@@ -210,6 +210,7 @@ npm run test:e2e
 │   ├── installation.md
 │   ├── requirements.md
 │   ├── design.md
+│   ├── component-structure.md
 │   ├── permissions.md
 │   ├── api-contract.md
 │   ├── features.md