Remove frontend JSON export actions

- Remove user-visible JSON export options from report editor, report management, template management, bulk template export, and AI debug logs.

- Keep HTML template package and PDF/browser print exports as the supported frontend export formats.

- Change per-template export to generate reusable HTML template packages.

- Preserve legacy JSON template import compatibility without exposing new JSON export buttons.

- Update README, AGENTS, feature, requirement, design, module, API contract, progress, and testing docs for the export policy change.

docs/api-contract.md

@@ -322,7 +322,7 @@ pageSize?: number
 
 ### `GET /api/reports/:id/export.json`
 
-返回结构化报告 JSON。权限与查看报告一致。当前确定不需要水印、导出原因、审批或专门导出审计；现阶段仍主要由前端 Blob 导出。
+返回结构化报告 JSON。权限与查看报告一致。当前确定不需要水印、导出原因、审批或专门导出审计；用户可见前端 Blob JSON 导出入口已移除，如后续需要结构化交换应由后端接口统一提供。
 
 ## Templates API