feat: side-by-side diff view + auto-save skips backup

- Side-by-side diff: toggle button (columns-2/align-justify icon) in toolbar
  switches between unified and side-by-side view
- Backend: diff endpoint returns left_content + right_content for side-by-side
- Frontend: SequenceMatcher-based LCS diff for two-column rendering
- CSS: .backup-diff-sidebyside with left/right borders, empty cell styling
- Auto-save: PUT /save?backup=false skips backup creation (auto-save=no backup)
- Footer text updated to reflect backup-on-manual-save behavior

frontend/style.css

@@ -7458,3 +7458,44 @@ body.popup-mode .content-area {
   font-size: 0.7rem;
   color: var(--text-muted);
 }
+
+/* ── Side-by-side diff view ── */
+.backup-diff-sidebyside .sidebyside {
+  table-layout: fixed;
+}
+
+.backup-diff-sidebyside td.sidebyside-left {
+  border-right: 1px solid var(--border);
+}
+
+.backup-diff-sidebyside td.sidebyside-right {
+  border-left: 1px solid var(--border);
+}
+
+.backup-diff-sidebyside td.diff-empty-cell {
+  background: var(--bg-tertiary);
+}
+
+.backup-diff-mode-btn {
+  padding: 6px 10px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  background: transparent;
+  color: var(--text-secondary);
+  cursor: pointer;
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  font-size: 0.8rem;
+  transition: all 150ms ease;
+}
+
+.backup-diff-mode-btn:hover {
+  color: var(--accent);
+  border-color: var(--accent);
+}
+
+.backup-diff-mode-btn .icon-sm {
+  width: 14px;
+  height: 14px;
+}