refactor(LLM config): Restructure LLM configuration and management

[mthezi]

WHAT:

• Support customizable LLM selection for each processing stage
• Optimize model selection based on task requirements

WHY:

• Enable cost optimization by using appropriate models for different tasks
• Improve translation quality with high-end models where needed

DETAILS:

• Recommend high-quality models (e.g. claude-3-5-sonnet) for translation tasks
• Suggest cost-effective models (e.g. Yi-Lightning) with JSON output support for simple operations
• Refactor config structure to support per-stage LLM configuration
• Update core modules to respect stage-specific LLM settings

AFFECTED:

• config.yaml: Add per-stage LLM configuration
• core/: Update LLM handling in processing modules
• st_components/: Update UI for LLM selection

Breaking Changes:

• Configuration file structure updated for LLM settings
• LLM initialization process modified