refactor(api): code-review fixes (R7 S1)

- localizeDetail: don't leak FastAPI's snake_case `loc` field names into a
  translated 422 toast; return a clean, fully localized validation message
  (drops the loc filter/pop parsing entirely)
- me(): drop the dead `r &&` guard now that req<T> types r non-nullable

frontend/src/i18n/locales/en/errors.json

@@ -1,7 +1,7 @@
 {
   "title": "Couldn't complete that",
   "generic": "The server couldn't carry out that action. Please try again.",
-  "validation": "Invalid value for “{{field}}”. Please check it and try again.",
+  "validation": "Some of the values you entered weren't valid. Please check them and try again.",
   "server": "Server error",
   "ok": "OK",
   "offline": {

frontend/src/i18n/locales/hu/errors.json

@@ -1,7 +1,7 @@
 {
   "title": "Nem sikerült",
   "generic": "A szerver nem tudta végrehajtani a műveletet. Próbáld újra.",
-  "validation": "Érvénytelen érték: „{{field}}”. Ellenőrizd, majd próbáld újra.",
+  "validation": "Néhány megadott érték érvénytelen volt. Ellenőrizd, majd próbáld újra.",
   "server": "Szerverhiba",
   "ok": "OK",
   "offline": {

frontend/src/lib/api.ts

@@ -228,15 +228,10 @@ class HttpError extends Error {
 function localizeDetail(d: unknown): string {
   const t = i18n.t.bind(i18n);
   // FastAPI request-validation errors: `detail` is an ARRAY of { loc, msg, type }. The UI sends
-  // well-formed requests, so this only fires on genuinely malformed input — surface the offending
-  // field name (localized) rather than FastAPI's raw English `msg`, which reads wrong in a HU UI.
-  if (Array.isArray(d)) {
-    const loc = (d[0] as { loc?: unknown } | undefined)?.loc;
-    const field = Array.isArray(loc)
-      ? loc.filter((s): s is string => typeof s === "string" && s !== "body").pop()
-      : undefined;
-    return field ? t("errors.validation", { field }) : t("errors.generic");
-  }
+  // well-formed requests, so this only fires on genuinely malformed input. The per-field `msg` and
+  // `loc` field names are English, snake_case internal tokens — surface a clean, fully localized
+  // "check your input" message rather than leaking them into a translated sentence.
+  if (Array.isArray(d)) return t("errors.validation");
   const o = d as { code?: string; reason?: string; limit?: number } | null | undefined;
   if (o?.code === "quota") {
     if (o.reason === "max_bytes") {
@@ -1117,7 +1112,7 @@ export const api = {
   me: async (): Promise<Me | null> => {
     // Always 200: `{ authenticated: false }` when logged out, else the full Me + `authenticated`.
     const r = await req<{ authenticated: boolean } & Partial<Me>>("/api/me");
-    return r && r.authenticated ? (r as Me) : null;
+    return r.authenticated ? (r as Me) : null;
   },
   accounts: (): Promise<Account[]> => req("/api/me/accounts"),
   deleteAccount: (): Promise<{ deleted: boolean }> => req("/api/me/account", { method: "DELETE" }),