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.bat

@@ -18,6 +18,19 @@ if %ERRORLEVEL% neq 0 (
     exit /b 1
 )
 
+REM Check if venv exists, create if not
+if not exist "venv" (
+    echo Creating virtual environment...
+    python -m venv venv
+)
+
+REM Activate the virtual environment
+call venv\Scripts\activate.bat
+
+REM Install dependencies
+echo Installing dependencies...
+pip install -r requirements.txt --quiet
+
 REM Run the Python launcher
 python "%~dp0start_ui.py" %*