Revert model dropdown and GPT-5 support for text generation

[Copilot]

This PR reverts the model dropdown and GPT-5 support functionality that was introduced in commit 4e6b20b. The application now uses a single fixed model for text generation instead of allowing users to select between different models.

Changes Made

Model Configuration

• Removed TEXT_MODELS array that contained selectable models including GPT-5
• Simplified to single default model: Now uses "anthropic/claude-4-sonnet" as the fixed text generation model
• Removed TextModel type definition as model selection is no longer needed

UI Components

• Removed model dropdown from the header component
• Cleaned up header props by removing selectedModel, onChangeModel, and availableModels parameters
• Removed Select component imports that were only used for model selection

State Management

• Removed model selection state from App.tsx including useQueryState("model")
• Removed localStorage persistence for selected model preferences
• Simplified Chat component to use fixed model instead of accepting selectedModel prop

API Routes

• Removed model parameter handling from request body and URL parameters
• Fixed model usage in createAIStream function to always use DEFAULT_TEXT_MODEL

Impact

• Bundle size reduced from 1.12MB to 1.04MB due to removed functionality
• Simplified user interface with no model selection dropdown in header
• Consistent model usage across all text generation requests
• No breaking changes to core chat functionality - users can still create and interact with chats normally

The revert maintains all existing functionality except for the ability to choose between different text generation models, effectively reverting to a simpler, single-model approach.

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