refactor(i18n): ask the catalogue for a sentence in one helper

`t()` answers a missing key with the key, so "does this code have a
sentence" was a comparison written inline at every site that wanted a
code's sentence or a fallback, twenty-odd times in several spellings.
translateOrNull, translateCodeOrNull and translateCode in
src/lib/i18n/translate-code.ts now hold that probe, every site uses
them, and a guard refuses the inline comparison anywhere else.

src/components/timeline/__tests__/no-template-key-guard.test.ts

@@ -24,8 +24,12 @@ import {
 
 const ROOTS = ["src/components/timeline", "src/components/day"] as const;
 
-/** `t(` or `tCount(` whose key is a template literal, across line breaks. */
-const TEMPLATE_KEY = /(?<![\w.])(?:t|tCount)\(\s*`/g;
+/**
+ * `t(`, `tCount(` or the catalogue probe `translateOrNull(t, ` whose key is
+ * a template literal, across line breaks.
+ */
+const TEMPLATE_KEY =
+  /(?<![\w.])(?:t|tCount)\(\s*`|(?<![\w.])translateOrNull\(\s*t\s*,\s*`/g;
 
 const ALLOWED: Readonly<Record<string, { count: number; why: string }>> = {
   "src/components/timeline/readiness-inventory.tsx": {

src/components/timeline/readiness-inventory.tsx

@@ -34,6 +34,7 @@ import {
 } from "./label-keys";
 import { formatMonthYear } from "./timeline-dates";
 import { orderedReadinessLanes, readinessTally } from "./readiness-model";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 type Translate = ReturnType<typeof useTranslations>["t"];
 type TranslateCount = ReturnType<typeof useTranslations>["tCount"];
@@ -65,11 +66,10 @@ export function wordKey(
   params: Record<string, string | number>,
 ): string | null {
   const count = typeof params.count === "number" ? params.count : null;
-  if (count !== null && t(`${base}Other`) !== `${base}Other`) {
+  if (count !== null && translateOrNull(t, `${base}Other`) !== null) {
     return tCount(base, count, params);
   }
-  const single = t(base, params);
-  return single === base ? null : single;
+  return translateOrNull(t, base, params);
 }
 
 export function laneLabel(t: Translate, key: TimelineReadinessLane["key"]) {