Add structured logging system with color-coded output and file logging (#1321)

## Summary
- Add comprehensive logging system with color-coded log levels ([INFO],
[WARN], [ERROR], [OK])
- Wrap all command executions with `run_cmd()` to capture output and
prevent leaks to stdout
- Add file logging with timestamps (default: `openvpn-install.log`)
- Suppress interactive prompts in auto-install mode for cleaner
CI/scripted usage
- Show log file location hint on errors for easier debugging

## Changes
- **openvpn-install.sh**: New logging functions (`log_info`, `log_warn`,
`log_error`, `log_fatal`, `log_success`, `log_prompt`, `log_header`,
`log_menu`, `run_cmd`), all `echo` statements converted to use logging
functions
- **test/validate-output.sh**: New E2E validator that ensures all script
output uses proper log formatting (catches raw echo leaks)
- **test/server-entrypoint.sh**: Integrates output validation into
Docker tests
- **test/Dockerfile.server**: Copies validation script into container

## Configuration
- `VERBOSE=1` - Show command output in terminal
- `LOG_FILE=path` - Customize log location (default:
`openvpn-install.log`)
- `LOG_FILE=""` - Disable file logging
- `FORCE_COLOR=1` - Force colored output in non-TTY environments

test/Dockerfile.server

@@ -35,7 +35,8 @@ RUN chmod +x /opt/openvpn-install.sh
 
 # Copy test scripts
 COPY test/server-entrypoint.sh /entrypoint.sh
-RUN chmod +x /entrypoint.sh
+COPY test/validate-output.sh /opt/test/validate-output.sh
+RUN chmod +x /entrypoint.sh /opt/test/validate-output.sh
 
 WORKDIR /opt