Installed apps: localize the "last seen" label (today / yesterday / N days ago) (#454)

formatLastSeen returned hard-coded English words ("today", "yesterday",
"N days ago") that were then substituted into the translated line
installed_playtime_line, so every non-English UI showed e.g. Spanish
"visto por ultima vez yesterday".

Format the day count with Intl.RelativeTimeFormat in the app language
(numeric: "auto"), so 0 / 1 days read as today / yesterday and their
equivalents in each locale (hoy / ayer / hace 5 dias, heute / gestern, ...).
This covers every shipped locale without new i18n keys.

- formatLastSeen takes an optional `locale` (default "en"), so existing
  callers and the English output are unchanged. An invalid tag falls back
  to the previous English strings.
- AppCard passes the active language from useLangStore.
- Tests: Spanish and German labels, and the invalid-tag fallback.

The "never" label (a title with play time but no last-seen timestamp) is
still English; it has no Intl equivalent and is left out of this change.

Not run locally: npm run validate / vitest / tsc / eslint (no Node toolchain
on the machine used). Intl outputs were checked in a browser console.

client/src/screens/InstalledApps/index.tsx

@@ -83,7 +83,7 @@ import { useMakeWay } from "../../lib/useMakeWay";
 import { pushNotification } from "../../state/notifications";
 import { withConsolePrefix } from "../../state/roster";
 import { audit } from "../../state/auditLog";
-import { useTr } from "../../state/lang";
+import { useLangStore, useTr } from "../../state/lang";
 import { transferAddr, mgmtAddr, hostOf } from "../../lib/addr";
 import { useImageRetry } from "../../lib/useImageRetry";
 import { transferScreenBusy } from "../../lib/ps5Transfers";
@@ -362,6 +362,7 @@ export const AppCard = memo(function AppCard({
    *  bookkeeping around a cold start doesn't run for a foreground nudge. */
 }) {
   const tr = useTr();
+  const lang = useLangStore((s) => s.lang);
   const navigate = useNavigate();
   const canPlay = !title.system;
   // On-console folder this title lives in — only folder/disc kinds carry a
@@ -493,7 +494,7 @@ export const AppCard = memo(function AppCard({
               {playSeconds && playSeconds > 0
                 ? tr(
                     "installed_playtime_line",
-                    { time: formatPlayTime(playSeconds), seen: formatLastSeen(lastSeenMs) },
+                    { time: formatPlayTime(playSeconds), seen: formatLastSeen(lastSeenMs, undefined, lang) },
                     `${formatPlayTime(playSeconds)} · last seen ${formatLastSeen(lastSeenMs)}`,
                   )
                 : tr("installed_playtime_never", undefined, "not seen playing")}

client/src/state/playTime.test.ts

@@ -153,6 +153,17 @@ describe("formatLastSeen", () => {
     expect(formatLastSeen(now - DAY, now)).toBe("yesterday");
     expect(formatLastSeen(now - 5 * DAY, now)).toBe("5 days ago");
   });
+  it("localizes the label to the app language", () => {
+    expect(formatLastSeen(now, now, "es")).toBe("hoy");
+    expect(formatLastSeen(now - DAY, now, "es")).toBe("ayer");
+    expect(formatLastSeen(now - 5 * DAY, now, "es")).toBe("hace 5 días");
+    expect(formatLastSeen(now - DAY, now, "de")).toBe("gestern");
+  });
+  it("falls back to English for an unknown locale tag", () => {
+    expect(formatLastSeen(now, now, "not a locale")).toBe("today");
+    expect(formatLastSeen(now - DAY, now, "not a locale")).toBe("yesterday");
+    expect(formatLastSeen(now - 5 * DAY, now, "not a locale")).toBe("5 days ago");
+  });
 });
 
 describe("formatPlayTime", () => {

client/src/state/playTime.ts

@@ -239,11 +239,26 @@ export function daysSinceLastSeen(
  *  "N days ago". Drives the Installed-Apps not-played surface (#116). The
  *  honest phrasing is "seen" — it's when ps5upload last observed the title
  *  running, not Sony's console-side last-played (which the app can't read
- *  cheaply). `now` injectable for tests. */
-export function formatLastSeen(ms: number | undefined, now = Date.now()): string {
+ *  cheaply). `now` injectable for tests. `locale` is the app language (a BCP-47
+ *  tag); it defaults to English so callers without one stay deterministic. */
+export function formatLastSeen(
+  ms: number | undefined,
+  now = Date.now(),
+  locale = "en",
+): string {
   const d = daysSinceLastSeen(ms, now);
   if (d === null) return "never";
-  if (d === 0) return "today";
-  if (d === 1) return "yesterday";
-  return `${d} days ago`;
+  try {
+    // `numeric: "auto"` turns 0 and -1 days into "today" / "yesterday" (and
+    // their equivalents in other languages); older reads "N days ago".
+    return new Intl.RelativeTimeFormat(locale, { numeric: "auto" }).format(
+      d === 0 ? 0 : -d,
+      "day",
+    );
+  } catch {
+    // Unknown locale tag: fall back to English.
+    if (d === 0) return "today";
+    if (d === 1) return "yesterday";
+    return `${d} days ago`;
+  }
 }