Add docs/V3_MIGRATION.md — a full write-up of the wailsv3 desktop migration (backend/frontend/build/CI changes, verification, branch map) and the planned follow-ups (toolchain majors + Android sideload). No behavior change.
Add a short notice to README.md: Migrating from v2 — v2 and v3 Secret Service collections coexist (AniTrack vs login) so both versions keep working after the first v3 re-login; the orphaned v2 entries are harmless. Details in docs/V3_MIGRATION.md. No cleanup instructions, by request.