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/package.json

@@ -9,6 +9,10 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@tiptap/extension-image": "^2.27.2",
+    "@tiptap/extension-link": "^2.27.2",
+    "@tiptap/react": "^2.27.2",
+    "@tiptap/starter-kit": "^2.27.2",
     "dompurify": "^3.4.11",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",