fix(i18n): write the locale to the account once per tab, not on every page

Every page load mounts the locale provider, and its mount effect sent
PUT /api/auth/me/locale with a value that had not changed. Every
authenticated write is charged to the account's shared write bucket, so
each page view spent one write. The mount write now happens once per tab
session and locale; a switch still writes at once.

src/lib/i18n/context.tsx

@@ -124,17 +124,39 @@ function persistLocale(newLocale: Locale) {
 // cooperation from the client. What this call still earns its place for is
 // the server-set cookie the route emits in reply: Safari's ITP caps the
 // script-written copy at 7 days, and a Set-Cookie is exempt.
+//
+// Once per tab session and per locale, not on every mount: every page load
+// mounts the provider, and every authenticated write is charged to the
+// account's shared write bucket, so a mount-time PUT spent one write per page
+// view on a value that had not changed. A switch still writes at once (the
+// locale differs from the one recorded), and a new tab writes once.
+const LOCALE_SYNCED_KEY = "healthlog-locale-synced";
+
 function persistLocaleToServer(newLocale: Locale) {
+  try {
+    if (sessionStorage.getItem(LOCALE_SYNCED_KEY) === newLocale) return;
+  } catch {
+    // Storage unavailable: write as before.
+  }
   void apiFetchRaw("/api/auth/me/locale", {
     method: "PUT",
     headers: { "Content-Type": "application/json" },
     body: JSON.stringify({ locale: newLocale }),
     credentials: "same-origin",
     keepalive: true,
-  }).catch(() => {
-    // Best-effort: the cookie remains the first-paint source of truth, and
-    // the next mount retries the backfill.
-  });
+  })
+    .then((res) => {
+      if (!res.ok) return;
+      try {
+        sessionStorage.setItem(LOCALE_SYNCED_KEY, newLocale);
+      } catch {
+        // Storage unavailable: the next mount writes again.
+      }
+    })
+    .catch(() => {
+      // Best-effort: the cookie remains the first-paint source of truth, and
+      // the next mount retries the backfill.
+    });
 }
 
 export function I18nProvider({
@@ -217,9 +239,10 @@ export function I18nProvider({
   }, [locale]);
 
   // v1.25.0 — keep `User.locale` in step with the committed UI locale. Runs on
-  // mount (backfilling the column for users who set their language before this
-  // landed) and on every switch, so the proactive Coach nudge and other
-  // cookie-blind background paths render in the language the user reads.
+  // the first mount of a tab session (backfilling the column for users who set
+  // their language before this landed) and on every switch, so the proactive
+  // Coach nudge and other cookie-blind background paths render in the language
+  // the user reads.
   useEffect(() => {
     persistLocaleToServer(locale);
   }, [locale]);