Add Fitbit API import flow to dashboard

[wellivea1]

This was done entirely in Codex with GPT 5.4. I apologize for any mistakes. This tool is necessary for my medical care and it was no longer being actively maintained, I might as well share this as it works.

Summary

• add a Fitbit API import dialog on the load screen
• implement browser-side OAuth PKCE and direct sleep import
• work around Fitbit sleep/list pagination gaps by restarting backfill queries with beforeDate
• route OAuth callbacks through the load view and keep the loading overlay active until info is ready

Why

Google/Fitbit export-based imports can be slow or incomplete. This adds a direct Fitbit API path for the hosted dashboard, including callback handling and UI behavior that starts loading immediately after redirect.

Notes

• this PR is intended for the hosted sleepdiary.github.io/dashboard flow
• the parser support for Fitbit API JSON lives in the paired core PR

Testing

• syntax-checked updated JS files locally
• manually tested the hosted-style redirect flow in the local dashboard
• verified that the post-auth redirect no longer requires clicking back to the load tab before import starts

[wellivea1]

This depends on the paired parser update in sleepdiary/core#52.

[wellivea1]

Added Google Health as an option due to the Fitbit API being deprecated this year. It has a working fallback in case you want the user to provide the Client ID.