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/styles/theme.css

@@ -312,6 +312,75 @@ button[disabled] {
   border: 1px solid var(--line);
 }
 
+/* ===== Rich text editor (TipTap) ===== */
+.rte {
+  border: 1px solid var(--line);
+  border-radius: 8px;
+  background: var(--bg);
+  overflow: hidden;
+}
+.rte:focus-within {
+  border-color: var(--accent);
+}
+.rte-toolbar {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 4px;
+  padding: 8px 10px;
+  border-bottom: 1px solid var(--line);
+  background: var(--panel-flat);
+}
+.rte-btn {
+  min-width: 30px;
+  height: 30px;
+  padding: 0 8px;
+  border: 1px solid transparent;
+  border-radius: 6px;
+  background: transparent;
+  color: var(--muted);
+  font-family: var(--sans);
+  font-size: 0.85rem;
+  line-height: 1;
+  cursor: pointer;
+  transition: background 0.12s, color 0.12s, border-color 0.12s;
+}
+.rte-btn:hover:not([disabled]) {
+  background: var(--blue);
+  color: var(--ink);
+}
+.rte-btn.is-active {
+  background: var(--blue);
+  border-color: var(--accent);
+  color: var(--accent-bright);
+}
+.rte-btn[disabled] {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+.rte-sep {
+  width: 1px;
+  align-self: stretch;
+  margin: 2px 4px;
+  background: var(--line);
+}
+.rte-content {
+  padding: 14px 16px;
+  max-height: 460px;
+  overflow-y: auto;
+}
+.rte-content .ProseMirror {
+  min-height: 220px;
+  outline: none;
+}
+.rte-content .ProseMirror p.is-editor-empty:first-child::before {
+  content: attr(data-placeholder);
+  float: left;
+  height: 0;
+  color: var(--dim);
+  pointer-events: none;
+}
+
 /* ===== Admin tables ===== */
 .adm-table {
   width: 100%;