Stop rendering the DocumentCenter as a blank page

Both the prose-documents and work-products sections rendered as
`{x.length > 0 ? ... : null}` with no else branch, and the whole
DocumentCenter was only mounted when a company was selected. The
combined result: a company with no files gave the operator a page
containing only a title and a "New" button, and an operator who had
not yet picked a company saw nothing at all — the exact "page is blank
even though docs were generated" complaint.

Add an explicit empty state inside DocumentCenter (surfaced only when
both collections are empty) and a separate "select a company" empty
state at the section router level. Strings live in the i18n bundle so
the Chinese locale is not left behind.

src/renderer/i18n/en.ts

@@ -873,6 +873,10 @@ export const en: Record<string, string> = {
   "ops.documentCenter": "Document Center",
   "ops.documentsProse": "Prose documents",
   "ops.documentsWorkProducts": "Files & deliverables",
+  "ops.documentsEmptyTitle": "No documents yet",
+  "ops.documentsEmptyDetail": "Agents register files here automatically when they write to the workspace, and can publish prose documents (PRDs, specs, post-mortems) via POST /api/companies/{id}/documents.",
+  "ops.documentsEmptyAction": "Create the first document",
+  "ops.documentsSelectCompany": "Select a company from the workspace switcher to see its documents.",
   "ops.knowledgeBase": "Knowledge Base",
   "ops.meetings": "Meetings",
   "ops.sprintBoard": "Sprint Board",

src/renderer/i18n/zh.ts

@@ -873,6 +873,10 @@ export const zh: Record<string, string> = {
   "ops.documentCenter": "文档中心",
   "ops.documentsProse": "文稿",
   "ops.documentsWorkProducts": "文件与交付物",
+  "ops.documentsEmptyTitle": "还没有文档",
+  "ops.documentsEmptyDetail": "Agent 在 workspace 里写文件时会被自动登记为交付物；结构化文稿（PRD、技术规格、复盘）需要调用 POST /api/companies/{id}/documents 发布。",
+  "ops.documentsEmptyAction": "创建第一份文档",
+  "ops.documentsSelectCompany": "请先在工作区切换器中选择一家公司，才能查看它的文档。",
   "ops.knowledgeBase": "知识库",
   "ops.meetings": "会议",
   "ops.sprintBoard": "迭代看板",