feat(forum): rebuild plate, post and compose pages

Match the reference layouts for the remaining forum routes: plate pages
get the blurred header card, moderator row and the affix tab bar with
per-tab filtering, topping posts and moderator-only pending/trash tabs;
post pages get the article shell with breadcrumbs, cover, meta row,
vote box, tag row and footer actions; compose pages get the reference
editor plus settings sidebar (submit/draft, status, access, plate,
topic, tag, type, vote) with a cover picker. Saving now honours draft
state and the featured image through the REST contract.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/admin-contract-report.json

@@ -1,5 +1,5 @@
 {
-    "generated_at": "2026-11-11T07:46:58+00:00",
+    "generated_at": "2026-11-11T08:16:48+00:00",
     "errors": [],
     "warnings": [
         "customPage tools-update section 'tools/update' not in PHP schema (ok if pure operational page)",