Support GitLab / Gitea / Bitbucket as vault hosts

[thetechjon]

Summary

noteser currently speaks one git host: GitHub. Competitive research (June 2026) shows the only comparable product (NotesHub) supports GitHub, GitLab, Bitbucket, and Gitea, and "single-vendor dependency" is a predictable objection from exactly the audience noteser targets.

Scope sketch

• Abstract src/utils/github.ts behind a host-agnostic Git Data interface (blobs/trees/commits/refs are near-isomorphic across hosts' REST APIs).
• Per-host OAuth/device-flow handling (Gitea/GitLab self-hosted instances need a base-URL setting).
• Keep the three-way merge engine untouched — it operates on SHAs and content, not on a specific host.

Priority

Roadmap item, not a bug. Worth doing AFTER the first distribution push, informed by which host people actually ask for.

[rotecodefraktion]

I need a codeberg implemantation, so technically a Forgejo/Gitea part. May I have a first mockup this week.