Add multi-language syntax highlighting to source editor, improve copy button to copy raw file content, and add permission error handling to file API endpoint

backend/main.py

@@ -527,7 +527,14 @@ async def api_file(vault_name: str, path: str = Query(..., description="Relative
     if not file_path.exists() or not file_path.is_file():
         raise HTTPException(status_code=404, detail=f"File not found: {path}")
 
-    raw = file_path.read_text(encoding="utf-8", errors="replace")
+    try:
+        raw = file_path.read_text(encoding="utf-8", errors="replace")
+    except PermissionError:
+        raise HTTPException(status_code=403, detail=f"Permission denied: cannot read file {path}")
+    except Exception as e:
+        logger.error(f"Error reading file {path}: {e}")
+        raise HTTPException(status_code=500, detail=f"Error reading file: {str(e)}")
+    
     ext = file_path.suffix.lower()
 
     if ext == ".md":