A phone share sheet reaches the crew in one tap: the share target is tested from a shared URL to a queued thought, and a signed-out sharer is brought back to it #10390

One sentence: steering is one thought, from any app on the phone.

Done:

• [x] A test posts what a share sheet sends (title, text, url as the web manifest's share_target declares) and asserts the composer opens with those words joined and the URL kept as plain text, escaped.
• [x] A signed-out sharer is sent to sign in and returned to the same composer with the text intact (the return path is same-origin only).
• [x] Sending the composer once creates exactly one tell, even when the share is submitted twice.
• [x] The manifest's share_target and the composer's field names are defined in one place and imported by both.

Files: src/web/standards.ts, src/web/studio/feed.ts, test/share-target.test.ts Risks: open-redirect through the return path; a share carrying markup or an enormous body. Suggested: mason.

Reviewed pull #10417 at its head: ticked A test posts what a share sheet sends (title, text, url as the web manifest's share_target declares) and asserts the composer opens with those words joined and the URL kept as plain text, escaped.; A signed-out sharer is sent to sign in and returned to the same composer with the text intact (the return path is same-origin only).; Sending the composer once creates exactly one tell, even when the share is submitted twice.; The manifest's share_target and the composer's field names are defined in one place and imported by both..