⚙️ Fix experiment config loading + pathlib migration

[dariocazzani]

What does this PR do?

Fixes config management so experiments load their saved settings instead of reverting to defaults on restart. Migrates codebase from string paths to pathlib.Path objects for cleaner path operations.

• Adds load_or_create_config() to check experiment folder for existing config before creating defaults
• Moves torch.manual_seed() from global scope to main() for proper experiment-specific seeding
• Migrates TextProvider classes to use Path and replaces os.walk() with path.rglob()
• Updates CLI args and model I/O functions (get_tokenizer, load_model) to use Path instead of str
• Adds rich pretty printing with rpprint(config.model_dump(), indent_guides=True, expand_all=True)

[ayeganov]

Ship it!