Fix archon:// protocol handler in Firefox extension

[macterra]

`archon://auth?challenge=...` links don't work in the Firefox extension.

Root cause: `chrome.action.openPopup(callback)` on lines 62 and 70 of `background.ts` uses Chrome's callback syntax. Firefox's `browser.action.openPopup()` takes no arguments and returns a Promise instead.

Fix: Replace callback-style `openPopup` calls with promise-based:
```js
await chrome.action.openPopup();
chrome.runtime.sendMessage({ ... });
```

Error message: `Incorrect argument types for action.openPopup`

🤖 Generated with Claude Code