feat: add MiniMax as first-class LLM provider with preset system

[octo-patch]

Summary

• Add a provider preset system to the LLM settings page — a dropdown lets users pick MiniMax, OpenAI, DeepSeek, or Ollama and auto-fills the endpoint URL, model list, and API-key hint
• New abogen/llm_providers.py module with LLMProviderPreset dataclass and 4 built-in presets (MiniMax at https://api.minimax.io/v1, OpenAI, DeepSeek, Ollama)
• Custom endpoints still work via the "Custom endpoint" option — no breaking changes

Changes

File	What changed
abogen/llm_providers.py	New module: LLMProviderPreset dataclass, built-in presets, get_provider_presets() and get_provider_by_id()
abogen/normalization_settings.py	Added llm_provider key to settings defaults
abogen/webui/templates/settings.html	Provider <select> dropdown in LLM settings section
abogen/webui/static/settings.js	applyProviderPreset() auto-fill logic + initProviderDropdown()
abogen/webui/routes/settings.py	Pass llm_provider_presets to template
abogen/webui/routes/utils/settings.py	Thread llm_provider through settings pipeline
README.md	Provider table and MiniMax configuration docs
.env.example	MiniMax Cloud commented-out example alongside Ollama

Test plan

• 10 unit tests for llm_providers.py (preset lookup, uniqueness, frozen dataclass, to_dict, etc.)
• 3 integration tests (settings pipeline round-trip, build_llm_configuration with MiniMax preset)
• All 13 new tests passing
• Manual: select MiniMax from dropdown -> verify base URL, models, and hint auto-fill
• Manual: select "Custom endpoint" -> verify fields are editable as before