Contribute rewrite Phase 4: §8.10 gutter accent in CM6 raw pane

Restores the §8.10 paragraph-margin marker layer Phase 1 dropped when
Tiptap left, this time against the CM6 raw pane on the left half of
the Contribute split. The matching inline tracked-insert/tracked-
delete overlay Phase 3 shipped lives on the preview pane to the
right; the two visual layers answer different questions on the two
halves of the split — "did anything change in this region?" (gutter,
amber, scannable) vs. "what changed here?" (inline, green/red,
precise) — and are deliberately separate signals.

New file: frontend/src/components/diffGutterExtension.js. The
extension exposes a `setBaseline` StateEffect and a gutter that marks
every line whose text differs from the same-indexed line of the
baseline (the last server-confirmed branch body). Per-line, not
paragraph-block — CM6's natural unit; collapsing to paragraph ranges
is more spec-faithful but adds code, and the per-line stance is the
pre-fancy default. A TODO is left for a future paragraph-collapse
pass if the result reads noisy.

MarkdownSourceEditor.jsx changes:
- Install the gutter extension in the editor's extension list.
- Seed the baseline to `initialDoc` at construct time.
- In the existing `initialDoc`-watching effect, dispatch the doc
  replacement AND a `setBaseline` effect in the SAME transaction so
  there's no one-frame window where the new doc reads as "divergent"
  against the old baseline. This carries through every server-
  confirmed branch refresh that RFCView already wires (accept,
  decline, manual flush, branch switch); no RFCView changes needed
  because all four paths already re-push `initialDoc` after pulling
  fresh state.

Design calls per the Phase 4 prompt's open list:
  • Per-line marks, not paragraph-block ranges. Pre-fancy stance.
  • Amber (#f59e0b) thin 3px vertical bar, distinct from the
    preview pane's green-on-light / red-on-light tracked-change
    inline overlay. Reads as "in-flight / not yet on the server."
  • Baseline reset on every branchView refresh (accept / decline /
    manual flush / branch switch), matching RFCView's existing
    originalSourceLinesRef discipline. Gutter then reads as "what's
    in the buffer but not on the server."
  • No hover / no click. The inline overlay already carries the
    click-to-card binding; the gutter is scannable only.

Known caveats kept deliberately:
  • Insert/delete shifts. Adding a line in the middle shifts every
    subsequent line's index and the naive compare lights up
    everything below — tolerated as the honest "you've touched
    stuff below this point" cue. A future LCS-anchored pass could
    fix it; Phase 4 doesn't need to.

SPEC §8.10:
- First paragraph rewritten to name the CM6 raw pane as the gutter's
  home and replace the stale "ProseMirror plugin" wording with the
  CodeMirror gutter framing. Mirrors how Phase 3 named the preview
  pane as the inline overlay's home.
- DiffView retirement paragraph adjusted: gutter and inline overlay
  together (across the split) cover its review affordance, not
  "both layers in the same surface" — the layers are deliberately on
  different surfaces.

Verification:
- Vite preview sandbox eval — standalone CM6 mount, dispatch tests
  across construct (no marks on identity), per-line edit (mark on
  exactly the touched line, confirmed by Y-coordinate matching the
  line-number gutter element), baseline reset clears, atomic
  doc+baseline dispatch leaves zero marks (the RFCView accept-flow
  path), insert-in-middle exhibits the expected cascade, and
  computed-style proof for the amber bar (`#f59e0b`, 3px wide,
  positioned left of the line-numbers gutter at x=21 vs x=24).
- Screenshot captures the bar on the touched line only.
- 125 backend integration tests still green.
- Live RFCView accept → branch refresh → gutter clear flow against a
  real branch was not driven (backend not running this session,
  carrying forward Phase 2/3's verification gap). The sandbox-level
  proof covers the atomic dispatch correctness; the next session
  with a backend up should drive that golden path before piling
  Phase 5 work on top.

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

frontend/src/App.css

@@ -580,6 +580,23 @@
 .cm-source-editor .cm-activeLineGutter { background: #f3f4f6; color: #555; }
 .cm-source-editor .cm-activeLine { background: #fafbfc; }
 
+/* §8.10 paragraph-margin marker (Phase 4) — gutter accent on lines
+   that differ from the open-session baseline (last server-confirmed
+   body). A scannable "did anything change in this region?" cue,
+   visually distinct from the preview pane's green/red tracked-insert
+   / tracked-delete overlay. Amber signals in-flight / uncommitted. */
+.cm-source-editor .cm-diff-gutter {
+  width: 3px; padding: 0;
+  background: transparent;
+}
+.cm-source-editor .cm-diff-gutter .cm-gutterElement {
+  padding: 0;
+}
+.cm-source-editor .cm-diff-gutter-mark {
+  width: 3px; height: 100%;
+  background: #f59e0b;
+}
+
 .readonly-bar {
   border-top: 1px solid #e5e5e5;
   padding: 10px 16px; text-align: center;