Close a dialog only on a press of its scrim

[BarganConstantin]

What changes

• Every dialog's backdrop now closes only on a press that starts and ends on the scrim, through the useScrimDismiss helper the feedback dialog got in Feedback dialog: keep the draft on a stray release, lock it while sending, and cancel on close #1925. Before this, a text selection begun in a field and let go past the dialog's edge arrived as a click on the backdrop and closed the dialog, throwing away what it held — a name typed into "This deck on the network", a share half pasted into Add account, the share Share accounts had just made. The 22 other backdrops (including the "did not load" dialog from Fix stale dialog chunks, fast-mode pricing in Projects, the offline tab icon, the phone topbar and two re-sign-in prompt cases #1929) spread the helper's handlers in place of onClick. Escape, the × and every close button are unchanged.
• Text-pinning tests that named the old onClick={…} on a backdrop are repointed to the new spread, and two test mocks of use-modal-dismiss now keep the real useScrimDismiss.

Verification

• npm run typecheck clean.
• Full suite: 870 files / 11501 tests passed (--maxWorkers=3 --minWorkers=1).
• New src/web/__tests__/dialog-scrim-dismiss.test.ts:
  • runs "This deck on the network", Add account and Share accounts on the suite's fake React and presses their backdrop the way the browser does: a selection let go over the scrim and a press taken back into the dialog both leave it open; a press of the scrim closes it, also right after a selection. All nine failed before the change (the dialog closed).
  • a source sweep of every modal-backdrop in the client: none has an onClick, each spreads a value made by useScrimDismiss(…). It failed before the change.
• Browser, isolated deck (roster, LAN status and the share request answered by the test page, nothing written), for Add account (paste field), Share accounts (with a share made) and This deck on the network (unsaved name):
  • drag-select from the field out past the dialog's edge, release over the scrim: the release's click lands on the backdrop, and the dialog stays open with its text;
  • press on the scrim, release inside the dialog: stays open;
  • click on the scrim: closes;
  • Escape: closes;
  • no page errors.