feat: add model switching during active chat session

[KingingWang]

Summary

Currently, users can only select a model when creating a new chat session. Once a session is active, there is no way to change the model without starting a completely new session.

Current Behavior

• Model selection is only available in ProviderSelectionEmptyState when !selectedSession && !currentSessionId (new session flow)
• Once a session exists, the UI only shows a "Continue conversation" prompt with no model selector
• The QuickSettingsPanel does not include model selection options

Desired Behavior

Add the ability to switch models during an active chat session, allowing users to:

1. Change the model mid-conversation without losing context
2. See a model selector/dropdown in the chat header or composer area
3. Have the new model take effect for subsequent messages

Suggested Implementation

• Add a model dropdown to ChatInputControls or a dedicated header component
• Handle model change via WebSocket message or new API endpoint
• Persist the model change to localStorage and session state

Context

This would improve the user experience for scenarios where:

• A user starts with a lighter model (Haiku/Sonnet) and wants to escalate to Opus for complex tasks
• A user wants to compare responses from different models on the same conversation
• A user realizes mid-session that a different model would be more appropriate

Related Files

• src/components/chat/view/subcomponents/ProviderSelectionEmptyState.tsx
• src/components/chat/hooks/useChatProviderState.ts
• src/components/chat/view/subcomponents/ChatComposer.tsx
• src/components/quick-settings-panel/view/QuickSettingsContent.tsx

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Show and switch active model in chat session #509

🔗 Related PRs

#301 - Feature/add thinking mode selector to chat interface [merged]
#374 - Refactor/app content main content and chat interface [merged]
#520 - fix: resolve duplicate key issue when rendering model options [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!