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

backend/main.py

@@ -1346,6 +1346,7 @@ async def api_file_save(
     vault_name: str,
     path: str = Query(..., description="Relative path to file"),
     body: dict = Body(...),
+    backup: bool = Query(True, description="Create a backup before saving (default true, set false for auto-save)"),
     current_user=Depends(require_auth),
 ):
     """Save (overwrite) a file's content.
@@ -1376,8 +1377,9 @@ async def api_file_save(
     content = body.get('content', '')
 
     try:
-        # Backup original content before overwriting
-        _backup_file(file_path, vault_name, path)
+        # Backup original content before overwriting (skip on auto-save)
+        if backup:
+            _backup_file(file_path, vault_name, path)
 
         file_path.write_text(content, encoding="utf-8")
         logger.info(f"File saved: {vault_name}/{path}")
@@ -2119,6 +2121,8 @@ async def api_file_diff(
             "version": version,
             "compare_with": compare_with,
             "diff": "".join(diff_lines),
+            "left_content": left_content,
+            "right_content": right_content,
         }
     except HTTPException:
         raise