Feedback dialog: keep the draft on a stray release, lock it while sending, and cancel on close

[BarganConstantin]

What changes

• A text selection let go over the scrim no longer closes the dialog. Dragging a selection out of the message and releasing past the dialog's edge reached the backdrop as a click and closed the dialog, losing the message, the contact and the screenshots. The backdrop now closes only on a press that goes down on the scrim and comes up there (useScrimDismiss in use-modal-dismiss.ts, ready for the other modals).
• The form holds still while a report sends. The request is built when Send is pressed, but paste, drop, the strip's remove and the message stayed live, so a screenshot removed mid-send still went and one pasted did not. Everything above Cancel and Send is now inert until the answer and no image is taken; focus in there moves to Send and comes back if the send fails. Cancel and the × stay live.
• Enter on Bug, Idea or Other no longer sends. A bare Enter on a radio submitted the form; it now does nothing, as in the contact field. The ⌘/Ctrl+Enter shortcut still sends from there.
• Closing while sending cancels. Cancel, Esc, the × or the scrim during "Sending…" only unmounted the dialog, and the report could still start uploading afterwards. A send still waiting on its images never posts, one under way is aborted, and neither reports anything afterwards.
• An image refused while Send waits stops the send. An image that failed its fit while Send waited was dropped, the report went without it and the thanks covered the refusal. Now the dialog stays open, the refusal stays beside the images, and a failure line says nothing was sent. A refusal shown before Send was pressed does not block sending.

Verification

• npm run typecheck: clean.
• Full suite (--maxWorkers=3 --minWorkers=1): 845 files, 11387 tests passed (exit 0). tarball-install-smoke.test.ts passed here too; it is known to time out in worktrees and pass in CI.
• New regression tests, each failing on development for the reason in the finding before its fix:
  • feedback-scrim-dismiss.test.ts: a press from the message released on the scrim called onClose (3 of 4 failed before).
  • feedback-sending-lock.test.ts: a paste and a drop during the send reached images.add, and the fields stayed live (4 of 6 failed before; two focus cases added with the fix).
  • feedback-kind-enter.test.ts: a bare Enter on a kind was not prevented.
  • feedback-send-cancel.test.ts: the post went out after unmount, and the fetch had no signal to abort.
  • feedback-refused-while-sending.test.ts: the report posted after its image was refused during the wait.
• fake-react.ts gains unmount() (runs effect cleanups; later state sets draw nothing). Two existing source pins were repointed for the backdrop's spread handlers and the Send button's ref.
• Browser (Chromium via Playwright, isolated deck, every POST /api/feedback answered by page.route): 22/22 checks passed. Drag-select over the scrim keeps the dialog and the text, and a plain scrim click still closes it. While sending, the fields are inert, the remove × and a paste change nothing, Cancel stays live, focus goes to Send and comes back to the message after a failure. Enter on a kind posts nothing, and Ctrl+Enter from it posts kind idea. Cancel during an upload aborts the request, and Esc while an image is resizing posts nothing. A damaged PNG refused while Send waits posts nothing and shows the failure and the refusal, and Send again posts the remaining image.