Move API calls to background script

[bttf]

Changes

• All backend API calls are made from background.ts
• Message passing is facilitated between useApi <> content_script <> background
• Error handling more refined, using ErrorCode to denote specific error types, and custom messaging and CTAs for each
• When no API key is found, we prompt users to enter it via the Options page (and provide a link to open it).
  • This is a temporary stopgap until we merge https://github.com/growthbook/growthbook/pull/1493 which should streamline the entire flow.

Notes

• Completely backwards compatible; can be shipped today with existing GB app
• GB PR https://github.com/growthbook/growthbook/pull/1493 will provide additional improvements, making updates more reliable

Tested (against both main branch and new 'streamlined' branch of GB app)

• [x] Initial load of visual changeset (happy path)
• [x] Initial load of visual changeset (error path)
• [x] Update visual changeset (happy path)
• [x] Update visual changeset (error path)
• [x] AI copy suggestion (happy path)
• [x] AI copy suggestion (error path)
• [x] No API key flow

[bttf]

Updates:

• Move apiHost to initial message payload (changes here)
• Show refined error msg for missing api key/host (backwards compat); pictured below
• Ensure we clean up deprecated apiHost query param (backwards compat)