docs(plan): mobile Share-to-shoplit (PWA share target) implementation plan

11 tasks: OAuth next= return-to (Go), vitest setup, share-text parser, PWA
manifest+SW+icons, addSharedItem client, /add page + form, mobile entry points,
e2e verification. No backend changes beyond the OAuth return-to; images stay
optional-URL + placeholder (upload deferred).

Author: mayur-tolexo