Wiki Phase 2: TipTap rich-text editor + inline image uploads

Replaces the raw-HTML textarea in the wiki editor with a TipTap (ProseMirror)
WYSIWYG editor.

- new RichTextEditor component: bold/italic/strike, H2/H3, bullet+ordered
  lists, blockquote, code block, divider, link, inline image, undo/redo
- generalized POST /admin/uploads (reuses the screenshot multer config) →
  { url }; the editor uploads inline images through it
- editor output still passes through the Phase 1 server-side sanitizer on
  save and DOMPurify on render
- lazy-loaded as its own chunk so the public bundle doesn't ship TipTap
  (public ~82kB gzip; editor chunk ~106kB gzip loaded only in admin)
- RTE styling added to theme.css (toolbar, active states, prose content)

Verified: uploads serve as images; H2/H3 + lists + link + inline image
round-trip through the WYSIWYG and render sanitized on the public page.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

website/WIKI_UPGRADE.md

@@ -299,10 +299,15 @@ phase if preferred). Do not merge a phase that hasn't been verified.
   index is data-driven (categories + sections); article shows category breadcrumb;
   client builds and server boots with no errors.
 
-### Phase 2 — Authoring UX
+### Phase 2 — Authoring UX ✅
 - TipTap editor replaces the textarea; generalized `/admin/uploads`; inline images.
 - **Exit check**: create/edit a page with headings, a list, a link, and an inline
   image; verify it renders sanitized on the public page.
+- **Verified** (2026-06-27): `/admin/uploads` returns `{url}` and the file serves as
+  an image; a page authored with H2/H3, lists, a link, and an uploaded inline image
+  round-trips through the WYSIWYG and renders sanitized publicly (link `rel` forced,
+  `<script>` stripped); a toolbar edit (insert divider) saved and persisted. TipTap
+  is code-split into its own chunk (lazy-loaded), keeping it off the public bundle.
 
 ### Phase 3 — Connectivity
 - Internal `[[slug]]` links + red-link detection; `wiki_links` rebuild on save;