Improve communication around CLI setup and PATH modifications

Context

As discussed in #1927, ToolHive Studio currently modifies shell dotfiles (e.g., .bashrc, .zshrc) to add the thv CLI to the user's PATH — without any visible notification or user consent.

This can be surprising, especially for users who carefully manage their dotfiles (e.g., via version control).

Problem

There is currently no communication to the user about:

• What ToolHive Studio is doing to their shell configuration
• That the thv CLI exists and is being installed
• How to manage or undo the changes

What we should explore

• Better surface what's happening during CLI setup (e.g., a dialog, a notification, a first-launch message)
• Provide an option to opt out of automatic PATH modification
• Leverage the existing Settings > CLI page to give users visibility and control
• Use this as an opportunity to educate users about the thv CLI

The exact approach is open for discussion — the key goal is that users are informed and in control.

Start by reviewing the existing CLI setup flow and the Settings > CLI page to understand where PATH changes and user communication are handled. Done means an agreed approach informs users about the thv CLI and shell changes, provides control over automatic PATH modification, and explains how to manage or undo it.