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>

client/src/api/client.js

@@ -70,6 +70,12 @@ export const api = {
       fd.append('image', file)
       return req('/admin/posts/upload', { method: 'POST', body: fd, raw: true })
     },
+    // Generalized upload for rich-text editors → { url }.
+    upload: (file) => {
+      const fd = new FormData()
+      fd.append('image', file)
+      return req('/admin/uploads', { method: 'POST', body: fd, raw: true })
+    },
     listWiki: (params = '') => req(`/admin/wiki${params}`),
     getWiki: (slug) => req(`/admin/wiki/${slug}`),
     createWiki: (data) => req('/admin/wiki', { method: 'POST', body: data }),