refactor(chat): consolidate slash command execution into message pipeline

[andrasbacsai]

Summary

• Move slash command execution from immediate handler to message sending pipeline
• Commands now insert into chat input as /commandname instead of executing immediately, allowing users to provide arguments
• Auto-detect and set pending command when user types space after a command name
• Clear pending commands when input is edited or form is submitted
• Add pendingCommands state to chat store to track selected command per session
• Simplify usePendingAttachments hook by removing command execution logic
• Update version to 0.1.31

Changes by File

ChatInput.tsx: Refactor command selection to insert into input rather than execute; auto-detect command when user types space after command name; clear pending command when input changes.

useMessageSending.ts: Add command resolution logic to handleSubmit; when a pending command exists, resolve it asynchronously and send resolved content plus user arguments as the message.

usePendingAttachments.ts: Remove handleCommandExecute and associated parameters; simplify to only handle attachment removal and queue management.

ChatWindow.tsx: Remove onCommandExecute prop from ChatInput; pass only essential parameters to usePendingAttachments.

chat-store.ts: Add pendingCommands state and setPendingCommand action to track slash commands per session.