feat: Add PlantUML syntax validation with auto-retry

When AI generates PlantUML code, run `plantuml -syntax` to validate.
If syntax errors are found, the error message is fed back to the AI
which retries with context about what went wrong.

This catches actual syntax errors like:
- Missing @startuml/@enduml tags
- Malformed arrows
- Invalid element definitions

Note: PlantUML's syntax checker only catches true syntax errors,
not semantic issues (like nesting rectangles in components).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/artifact_editor/gui.py

@@ -252,11 +252,56 @@ class AIThread(QThread):
 
     def _validate_output(self, code: str) -> tuple:
         """Validate the generated code. Returns (is_valid, error_message)."""
+        # For PlantUML, run syntax check via plantuml command
+        if self.artifact_type == 'plantuml':
+            return self._validate_plantuml(code)
+
+        # For other formats, use renderer validation
         renderer = self._get_renderer()
         if renderer:
             return renderer.validate(code)
         return True, None  # No validation available
 
+    def _validate_plantuml(self, code: str) -> tuple:
+        """Validate PlantUML code using plantuml -syntax command."""
+        import shutil
+
+        plantuml_cmd = shutil.which('plantuml')
+        if not plantuml_cmd:
+            # PlantUML not installed, skip validation
+            return True, None
+
+        try:
+            result = subprocess.run(
+                [plantuml_cmd, '-syntax', '-pipe'],
+                input=code,
+                capture_output=True,
+                text=True,
+                timeout=30
+            )
+
+            # Exit code 0 = valid, anything else = error
+            if result.returncode == 0:
+                return True, None
+
+            # Parse error message from stdout
+            # Format: "ERROR\n<line_number>\nSyntax Error?\n<description>"
+            error_lines = result.stdout.strip().split('\n')
+            error_msg = "PlantUML syntax error"
+
+            if len(error_lines) >= 2 and error_lines[0] == 'ERROR':
+                line_num = error_lines[1]
+                error_msg = f"PlantUML syntax error on line {line_num}"
+                if len(error_lines) >= 4:
+                    error_msg += f": {error_lines[3]}"
+
+            return False, error_msg
+
+        except subprocess.TimeoutExpired:
+            return True, None  # Skip validation on timeout
+        except Exception as e:
+            return True, None  # Skip validation on error
+
     def _call_ai(self, instruction: str, current_code: str) -> tuple:
         """Call artifact-ai SmartTool and return (success, response_or_error).