fix(web): keep the plan card to decisions it can actually answer

Review follow-ups on the presentation intent.

The card claimed any single-question request declaring the intent, then sent one
of two labels — so a producer offering a third option, or a multi-select batch,
lost answers the generic flow would have shown. That contradicts the intent's
own contract, so `planReviewOf` now claims only a binary single choice and
leaves everything else to the flow that can express it.

`ask()` also rejects a plan-review intent on a question with no `detail`: the
intent declares detail IS the plan, and without one a honouring UI asks the
user to approve something invisible. The client keeps its own fallback — it sits
downstream of a wire boundary — but the misconfiguration now fails at the asker.

`planReviewOf` stops being a value export of the client contract face (client
export discipline: pure helpers stay internal; the tests already import it
relatively), and the ui-question README fallback list, both languages, now
states every condition the code enforces.

packages/ui/user-interaction/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/ui/user-interaction/README.md
-README.md: f134e912149190efb81ea9f64c5d63632ce8c626
-README.zh.md: ab6432dcbddc36d3ef9ca957b3901cb76257539a
+README.md: d62e75d110b8be339c5f9449b0834320f695ac99
+README.zh.md: 55258e85e56df2375ed8f195fa0b3b731a9cb816