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/js/utils.js

@@ -437,7 +437,8 @@ async function saveFile(silent = false) {
       safeCreateIcons();
     }
 
-    const response = await fetch(`/api/file/${encodeURIComponent(state.editorVault)}/save?path=${encodeURIComponent(state.editorPath)}`, {
+    const backupParam = silent ? '&backup=false' : '';
+    const response = await fetch(`/api/file/${encodeURIComponent(state.editorVault)}/save?path=${encodeURIComponent(state.editorPath)}${backupParam}`, {
       method: "PUT",
       headers: { "Content-Type": "application/json" },
       body: JSON.stringify({ content }),