Replace environment variable config with CLI flag configuration

- Migrate from environment variables to CLI flags for configuration
- Add support for --listen, --root-path, --dir-indexing, --log-level, --response-timeout flags
- Remove config validation error struct as it's no longer needed
- Update .gitignore to exclude /dist directory
- Simplify configuration loading with flag.Parse()

.gitignore

@@ -2,3 +2,4 @@
 **/*~
 /.idea
 /run.sh
+/dist