Contribute rewrite Phase 7: retire DiffView

The Phase 3 preview-pane inline overlay and Phase 4 raw-pane gutter
accent together cover the review affordance DiffView was the interim
home for, so the toolbar toggle collapses and the surface goes.

- frontend/src/components/DiffView.jsx deleted.
- RFCView.jsx: drop the DiffView + marked imports, the reviewMode /
  reviewHTML state, the toggleReviewMode callback, the "Review changes"
  toolbar button, and the reviewMode ternary in render. editorEditable
  simplifies to mode === 'contribute' && canContribute. File-header
  docstring updated: "DiffView" → "preview-pane tracked-change overlay
  (§8.10)".
- App.css: remove the .diff-view-wrapper and .diff-view-empty rules;
  relabel the DiffView section header as ChangeTooltip (which lives on
  as the Phase 3 overlay's hover affordance). Drop the dead .editor-
  content .tiptap rules — Tiptap had no remaining consumers after
  DiffView's deletion. The Contribute toolbar still renders the
  accepted/pending hint + Phase 6 Chat toggle; only the Review-changes
  button is removed.
- MarkdownPreview.jsx: comment updated to drop the DiffView mention
  from the scoped-Marked-instance rationale.
- SPEC.md: §8.10 retitled "Tracked-change markup" (was "...and the
  review-mode toggle"); the legacy-DiffView paragraph rewritten as a
  one-sentence retirement note. §8.1, §8.15, §9.4, §9.5 references to
  DiffView / review-mode cleaned up. §19.2's persistent-accepted-change
  candidate updated to point at the preview-pane overlay rather than
  DiffView.

No DiffView orphans remain (grep confirms .btn-review-toggle is still
the Discuss-mode "Show tracked changes" toggle, ChangeTooltip and
trackedOverlay carry historical comments only, PRView's §19.2 comment
is unrelated). Frontend reload shows no console errors.

Backend integration suite: 125 passed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/App.css

@@ -431,31 +431,6 @@
 .editor-content {
   max-width: 720px; margin: 0 auto; outline: none;
 }
-.editor-content .tiptap {
-  outline: none; font-size: 15px; line-height: 1.75; color: #1a1a1a;
-}
-.editor-content .tiptap h1 { font-size: 22px; font-weight: 700; margin: 24px 0 12px; }
-.editor-content .tiptap h2 { font-size: 17px; font-weight: 600; margin: 20px 0 8px; }
-.editor-content .tiptap h3 { font-size: 15px; font-weight: 600; margin: 16px 0 6px; }
-.editor-content .tiptap p { margin: 0 0 12px; }
-.editor-content .tiptap ul, .editor-content .tiptap ol { padding-left: 24px; }
-.editor-content .tiptap code { background: #f0f0ee; padding: 1px 5px; border-radius: 3px; font-size: 13px; }
-
-.editor-content .tiptap .selection-highlight {
-  background: rgba(99, 102, 241, 0.15);
-  border-radius: 2px;
-  outline: 1px solid rgba(99, 102, 241, 0.3);
-  outline-offset: 1px;
-}
-.editor-content .tiptap .tracked-delete {
-  background: #fee2e2; color: #991b1b; text-decoration: line-through;
-  border-radius: 2px; padding: 1px 2px; cursor: pointer;
-}
-.editor-content .tiptap .tracked-insert {
-  background: #dcfce7; color: #166534;
-  border-radius: 2px; padding: 1px 2px; cursor: pointer;
-}
-
 /* CodeMirror 6 source editor (Contribute mode). */
 .cm-source-editor {
   flex: 1; min-height: 0;
@@ -540,11 +515,11 @@
   background: rgba(99, 102, 241, 0.22);
 }
 
-/* Tracked-change overlay (Phase 3, §8.10) — preview-pane equivalents of
-   the .editor-content .tiptap rules above. The Contribute pane shows
-   these by default; the Discuss pane gates them behind a toolbar
-   toggle. Hovering surfaces a ChangeTooltip with the source message
-   and reason. */
+/* Tracked-change overlay (Phase 3, §8.10) — green/red decorations on
+   accepted-change spans inside the rendered preview. The Contribute
+   pane shows these by default; the Discuss pane gates them behind a
+   toolbar toggle. Hovering surfaces a ChangeTooltip with the source
+   message and reason. */
 .markdown-preview .tracked-delete {
   background: #fee2e2; color: #991b1b; text-decoration: line-through;
   border-radius: 2px; padding: 1px 2px; cursor: pointer;
@@ -1057,16 +1032,9 @@
   font-size: 13px; font-weight: 600; cursor: pointer;
 }
 
-/* ── DiffView ──────────────────────────────────────────────────── */
+/* ── ChangeTooltip (shared between the Phase 3 preview-pane overlay
+   and the §8.10 hover affordance) ─────────────────────────────────── */
 
-.diff-view-wrapper {
-  flex: 1; overflow-y: auto;
-  padding: 32px 48px;
-}
-.diff-view-empty {
-  font-size: 13px; color: #999;
-  text-align: center; padding: 24px;
-}
 .diff-tooltip {
   background: #fff; border: 1px solid #e5e5e5;
   border-radius: 8px; padding: 10px 12px;