feat: Jan supports multiple assistants

[louis-menlo]

Describe Your Changes

This pull request introduces functionality to manage and utilize a "current assistant" across various parts of the application. Key changes include updates to the assistant and thread state management, integration of assistant-specific instructions in chat messages, and enhancements to the CompletionMessagesBuilder class.

CleanShot.2025-05-20.at.00.18.48.mp4

Closes #2845

Assistant State Management

• Added a currentAssistant property and corresponding setCurrentAssistant method to the useAssistant hook for tracking and updating the currently selected assistant. (web-app/src/hooks/useAssistant.ts, [1] [2] [3]

Thread State Management

• Introduced an updateCurrentThreadAssistant method in the useThreads hook to associate the current assistant with the active thread. (web-app/src/hooks/useThreads.ts, [1] [2]

Chat Integration

• Updated the useChat hook to include assistant-specific instructions in chat messages by leveraging the currentAssistant property. (web-app/src/hooks/useChat.ts, [1] [2] [3]

UI Updates

• Modified the DropdownAssistant component to use currentAssistant for selection and ensure thread updates when switching assistants. (web-app/src/containers/DropdownAssistant.tsx, [1] [2]
• Updated the Index and ThreadDetail components to synchronize the current assistant with the active thread and reset the thread ID when navigating. (web-app/src/routes/index.tsx, [1] [2] [3]; web-app/src/routes/threads/$threadId.tsx, [4] [5] [6]

Code Enhancements

• Enhanced the CompletionMessagesBuilder class with a new addSystemMessage method and improved documentation for better maintainability. (web-app/src/lib/messages.ts, [1] [2]

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed