fix: activate venv in UI launcher scripts

start_ui.sh and start_ui.bat were using system Python directly,
causing ModuleNotFoundError for dotenv. Now they create and activate
the venv before running, matching the pattern in start.sh.

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

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

start_ui.sh

@@ -21,5 +21,18 @@ else
     PYTHON_CMD="python3"
 fi
 
+# Check if venv exists, create if not
+if [ ! -d "venv" ]; then
+    echo "Creating virtual environment..."
+    $PYTHON_CMD -m venv venv
+fi
+
+# Activate the virtual environment
+source venv/bin/activate
+
+# Install dependencies
+echo "Installing dependencies..."
+pip install -r requirements.txt --quiet
+
 # Run the Python launcher
-$PYTHON_CMD start_ui.py "$@"
+python start_ui.py "$@"