feat: implement Copilot provider integration with token storage and login flow

[roshan-c]

This PR adds full support for tracking GitHub Copilot usage statistics in CodexBar. It implements a secure authentication flow and fetches quota data from the internal GitHub Copilot API.

[steipete]

I tried this, but it's not working as intended. I'll take a look.

[steipete]

codex analysis:

• Yes — not closing is expected with current flow; we have two real bugs:

• waitingAlert.runModal() blocks, and we never close the alert window; NSApp.stopModal alone
  doesn’t reliably dismiss the NSAlert.
• We start two polling tasks; cancel only stops the unused one. The real poll keeps running
  (or failing), so the modal never ends.

Likely also failing to ever get a token: the device‑flow endpoints expect form‑urlencoded
bodies, but we send JSON. That can leave polling stuck even though the web page says
success.

Fix plan (small, safe):

1. CopilotDeviceFlow: send application/x-www-form-urlencoded bodies for both device code +
   token. Keep Accept: application/json.
2. CopilotLoginFlow: single polling task; on success call NSApp.stopModal and
   waitingAlert.window.close() (or switch to beginSheetModal).
3. Cancel should cancel the same task.

[steipete]

Thanks @roshan-c! We merged this into main and added a credit in the CHANGELOG.

[greptile-apps]

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