feat(docgov): expose proposal HTTP and wire frontend decide flow

Add POST /api/v1/docgov/proposal for ProposeModification, harden decide
status mapping, and replace App pendingProposal local mock with real API calls.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/server/main.go

@@ -70,6 +70,7 @@ func main() {
 	fmt.Println("  • [GET]  /api/v1/context/cases/recommend - 往期受控标杆案例与历史报价语义自推荐")
 	fmt.Println("  • [GET]  /api/v1/docgov/document    - 查询受控文档治理状态看板")
 	fmt.Println("  • [POST] /api/v1/docgov/claim       - 查阅并认领受控文档 (正式稿方可认领)")
+	fmt.Println("  • [POST] /api/v1/docgov/proposal    - 对已认领文档发起变更提案")
 	fmt.Println("  • [POST] /api/v1/docgov/proposal/decide - 编制负责人拍板同意/拒绝变更提案")
 	fmt.Println("  • [POST] /api/v1/docgov/submit-review - 编制完成提交审核 (文档正文锁定)")
 	fmt.Println("  • [POST] /api/v1/docgov/review-decision - 审核通过或驳回 (驳回必填原因且解锁)")