fix: allow setting model type when adding custom models, preserve on restart

[octo-patch]

Fixes #271

Problem

When users add a new model via the settings dialog, the model type is hardcoded to text, making it impossible to configure image or video models for custom providers (such as OpenRouter). Additionally, if a user manually edited config.toml to set a model's type to image or video for a custom provider, those models were silently dropped on app restart because config_service.initialize() only preserved text-type user-added models for all providers.

Solution

Frontend (AddModelsList.tsx)

• Add a Model Type selector (text / image / video) to the Add Model dialog, defaulting to text to preserve existing behavior for LLM models.

Backend (config_service.py)

• In initialize(), distinguish between built-in providers (in DEFAULT_PROVIDERS_CONFIG) and custom providers (e.g. OpenRouter, any user-added provider).
• For built-in providers: keep the existing behavior — only user-added text models are preserved (image/video are hardcoded per-provider).
• For custom providers: preserve all model types as configured, so image/video models survive app restarts.
• Use dict() copy when reading DEFAULT_PROVIDERS_CONFIG models to avoid mutating the module-level default on repeated initialize() calls.

Testing

• Add a custom provider (e.g. OpenRouter) in Settings
• Click "Add Model", set name to an image-capable model, select type "image"
• Save settings and restart the app — the model type is now preserved
• Previously the model would revert to text or disappear entirely on restart