refactor(documents): split the document detail sheet

The sheet becomes a composer over detail/: the original-file preview and
download controls, the detail read with its writes, the AI layer with its
gate and review-first apply, and the field, meta and footer sections.
Guards that read the sheet's source now read the detail folder.

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 401,
-  "total": 138066,
+  "total": 137456,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -121,7 +121,7 @@
     "components/documents/document-ai-section.tsx :: DocumentAiSection": 262,
     "components/documents/document-bulk-bar.tsx :: DocumentBulkBar": 204,
     "components/documents/document-card.tsx :: DocumentCard": 354,
-    "components/documents/document-detail-sheet.tsx :: DocumentDetailSheet": 935,
+    "components/documents/document-detail-sheet.tsx :: DocumentDetailSheet": 325,
     "components/documents/document-facts-review.tsx :: DocumentFactsSection": 252,
     "components/documents/document-facts-review.tsx :: FactRow": 262,
     "components/documents/document-filter-bar.tsx :: DocumentFilterBar": 348,