Add settings page, PIN manager, and auto-config

- New settings page at /settings (localhost-only) accessible via tray icon
- PIN manager with 6-digit codes, 5-min TTL, single-use
- API token uses secrets.token_urlsafe(32) for 256-bit entropy
- Config auto-creates config.yaml on first run with detected local IP
- Default PIN and token generated randomly on first startup
- Added pin_ttl setting (default 300s)
- Updated build_exe.py for onefile exe with settings assets bundled
- Added start.bat and start.sh for easy development startup

server/start.bat

@@ -0,0 +1,33 @@
+@echo off
+REM PaperDash Server Startup Script
+REM Creates venv if needed, installs dependencies, and starts the server
+
+setlocal
+
+cd /d "%~dp0"
+
+REM Create venv if it doesn't exist
+if not exist "venv" (
+    echo Creating virtual environment...
+    python -m venv venv
+)
+
+REM Activate venv
+call venv\Scripts\activate.bat
+
+REM Install dependencies
+echo Installing dependencies...
+pip install --upgrade pip
+pip install -r requirements.txt
+
+REM Run the server
+echo Starting PaperDash server...
+echo   Dashboard: http://localhost:8921
+echo   API docs:  http://localhost:8921/docs
+echo.
+echo Set environment variables before starting:
+echo   DASHBOARD_PIN=1234
+echo   API_TOKEN=your-token
+echo.
+
+uvicorn main:app --host 0.0.0.0 --port 8921 --reload