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/insights/coach-panel/assumption-line.tsx

@@ -21,6 +21,7 @@ import type { CoachAssumption, CoachFollowUp } from "@/lib/ai/coach/types";
 
 import { SuggestedReplies } from "./suggested-replies";
 import { COACH_FOCUS_RING } from "./focus-ring";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 /** The alternatives the server offers for one assumption. */
 export function alternativesFor(
@@ -39,8 +40,7 @@ function optionLabel(
   t: (key: string) => string,
   option: CoachAssumption["value"],
 ): string {
-  const local = t(option.labelKey);
-  return local === option.labelKey ? option.label : local;
+  return translateOrNull(t, option.labelKey) ?? option.label;
 }
 
 export interface CoachAssumptionLineProps {

src/components/insights/coach-panel/chat-bubble.tsx

@@ -38,6 +38,7 @@ import type {
   CoachMemoryNote as CoachMemoryNoteFrame,
   CoachProvenanceMetric,
 } from "@/lib/ai/coach/types";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 /**
  * Per-message bubble renderer for the Coach thread, split out of
@@ -239,8 +240,8 @@ function coachChartTitle(
 ): string {
   const override = CHART_TITLE_KEY_OVERRIDE[metric];
   if (override) {
-    const resolved = t(override);
-    if (resolved !== override) return resolved;
+    const resolved = translateOrNull(t, override);
+    if (resolved !== null) return resolved;
   }
   const camel = metric
     .toLowerCase()
@@ -250,8 +251,7 @@ function coachChartTitle(
     )
     .join("");
   const key = `measurements.type${camel.charAt(0).toUpperCase()}${camel.slice(1)}`;
-  const resolved = t(key);
-  return resolved === key ? metric.replace(/_/g, " ").toLowerCase() : resolved;
+  return translateOrNull(t, key) ?? metric.replace(/_/g, " ").toLowerCase();
 }
 
 interface ChatBubbleProps {
@@ -564,16 +564,11 @@ function ChatBubbleImpl({
   // provider load (retry in ~5 min). Codes that have no dedicated
   // translation fall back to the generic provider-unavailable copy.
   const errorKey = errorCode ? errorCodeToI18nKey(errorCode) : null;
-  const errorMessage = errorKey ? t(errorKey, {}) : null;
-  // When a translated message comes back unchanged (i.e. key missing)
-  // we fall back to a generic provider error string so the bubble
-  // doesn't surface raw `coach.http.503` text to the user.
+  // A code without a sentence falls back to the generic provider error, so
+  // the bubble never surfaces raw `coach.http.503` text to the user.
   const safeError =
-    errorMessage && errorMessage !== errorKey
-      ? errorMessage
-      : errorCode
-        ? t("insights.coach.errorProvider")
-        : null;
+    (errorKey ? translateOrNull(t, errorKey) : null) ??
+    (errorCode ? t("insights.coach.errorProvider") : null);
 
   const method = metricSource?.method ?? null;
 

src/components/insights/coach-panel/coach-conversation.tsx

@@ -67,6 +67,7 @@ import {
   useDetachCoachDocument,
   useSendCoachMessage,
 } from "./use-coach";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 /**
  * v1.12.0 (Coach v2 #6) — shared chat surface for both the Coach drawer
@@ -987,11 +988,8 @@ export function CoachConversation({
   // from a metric surface names the metric there until its first turn.
   const a2Metric = launchScope?.metric ?? null;
 
-  // `t()` returns the key string itself when a key is missing, so the only
-  // reliable "is this key defined" signal is `t(key) !== key`.
   function tOrNull(key: string): string | null {
-    const resolved = t(key);
-    return resolved === key ? null : resolved;
+    return translateOrNull(t, key);
   }
 
   // The resolved metric label: prefers a per-source i18n key, falls back to

src/components/insights/coach-panel/message-thread.tsx

@@ -42,6 +42,7 @@ import {
   COACH_MEMORY_KEYS,
   COACH_PLAN_KEYS,
 } from "@/lib/ai/coach/memory/shared";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 // v1.28.26 file-size split (pure code motion): the bubble renderer +
 // per-message actions live in `chat-bubble.tsx`, the read-aloud stack in
@@ -215,8 +216,7 @@ export function decisionReplies(
       ]
     : choices;
   for (const choice of ordered.slice(0, 4)) {
-    const local = t(choice.labelKey);
-    const label = local === choice.labelKey ? choice.label : local;
+    const label = translateOrNull(t, choice.labelKey) ?? choice.label;
     out.push({
       id: `clarify-${choice.id}`,
       label,

src/components/insights/coach-panel/turn-activity.tsx

@@ -82,6 +82,7 @@ import type {
 
 import { COACH_FOCUS_RING } from "./focus-ring";
 import { CoachMethodLine } from "./method-line";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 /** The shortest gap between two screen-reader announcements. */
 export const ANNOUNCE_THROTTLE_MS = 1500;
@@ -106,8 +107,7 @@ function tOr(
   fallback: string,
   params?: Record<string, string | number>,
 ) {
-  const value = t(key, params);
-  return value === key ? fallback : value;
+  return translateOrNull(t, key, params) ?? fallback;
 }
 
 // ── Steps (messages saved before v1.41, and the fetch rows) ────────────────
@@ -180,9 +180,8 @@ export function legacyAreaLabels(
   const out: string[] = [];
   for (const area of areas ?? []) {
     if (area === "general") continue;
-    const key = `insights.coach.metric.${area}`;
-    const label = t(key);
-    if (label === key || out.includes(label)) continue;
+    const label = translateOrNull(t, `insights.coach.metric.${area}`);
+    if (label === null || out.includes(label)) continue;
     out.push(label);
   }
   return out;

src/components/insights/workout-detail/header.tsx

@@ -7,6 +7,7 @@ import type { WorkoutDetailPayload } from "@/hooks/use-workouts";
 import { useUnitDisplay } from "@/hooks/use-unit-display";
 
 import { formatDuration, formatDistance } from "./format";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 /**
  * Hero header — the at-a-glance verdict row. Sport icon badge, source
@@ -34,9 +35,7 @@ export function WorkoutDetailHeader({ workout }: WorkoutDetailHeaderProps) {
   const fmt = useFormatters();
   const { preference } = useUnitDisplay();
   const sportLabelKey = `insights.workouts.sport.${workout.sportType}`;
-  const sportLabel = t(sportLabelKey);
-  const sportName =
-    sportLabel === sportLabelKey ? workout.sportType : sportLabel;
+  const sportName = translateOrNull(t, sportLabelKey) ?? workout.sportType;
 
   return (
     <header

src/components/insights/workout-list.tsx

@@ -13,6 +13,7 @@ import type { WorkoutListEntry } from "@/hooks/use-workouts";
 import { Button } from "@/components/ui/button";
 import { useUnitDisplay } from "@/hooks/use-unit-display";
 import { formatDistance } from "@/components/insights/workout-detail/format";
+import { translateOrNull } from "@/lib/i18n/translate-code";
 
 /**
  * v1.4.32 — workout list row primitive.
@@ -63,12 +64,11 @@ export function WorkoutList({
       {workouts.map((workout) => {
         const Icon = iconForSport(workout.sportType);
         const sportLabelKey = `insights.workouts.sport.${workout.sportType}`;
-        const sportLabel = t(sportLabelKey);
         // Translation keys missing from one locale fall back to the
         // canonical sport-type string so a new HK enum value never
         // renders the raw `insights.workouts.sport.x` placeholder.
         const sportName =
-          sportLabel === sportLabelKey ? workout.sportType : sportLabel;
+          translateOrNull(t, sportLabelKey) ?? workout.sportType;
 
         return (
           // v1.42 — the row opens the workout and its date opens the day: