Fixes model and usage status getting mixed up between sessions.
- Each session now keeps its own worker model; picking a worker in one session no longer rewrites every other open session, and reconnects restore the session's own worker (#54).
- The footer's weekly-limit warning now follows the selected session's main model provider: GPT sessions show the ChatGPT weekly limit for the account actually serving them, instead of Grok's (#55).
Thanks to @chrislong365 for both fixes.