diff --git a/frontend/src/i18n/locales/en/errors.json b/frontend/src/i18n/locales/en/errors.json
index 56f38c7..8a8ec37 100644
--- a/frontend/src/i18n/locales/en/errors.json
+++ b/frontend/src/i18n/locales/en/errors.json
@@ -1,6 +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.",
   "server": "Server error",
   "ok": "OK",
   "offline": {
diff --git a/frontend/src/i18n/locales/hu/errors.json b/frontend/src/i18n/locales/hu/errors.json
index 43f5311..4bd1342 100644
--- a/frontend/src/i18n/locales/hu/errors.json
+++ b/frontend/src/i18n/locales/hu/errors.json
@@ -1,6 +1,7 @@
 {
   "title": "Nem sikerült",
   "generic": "A szerver nem tudta végrehajtani a műveletet. Próbáld újra.",
+  "validation": "Érvénytelen érték: „{{field}}”. Ellenőrizd, majd próbáld újra.",
   "server": "Szerverhiba",
   "ok": "OK",
   "offline": {
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 0240262..509285f 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -212,8 +212,8 @@ export interface VersionInfo {
 class HttpError extends Error {
   status: number;
   detail?: string; // server-provided reason (FastAPI's `detail`), when present
-  detailData?: any; // structured detail ({ code, reason, ... }) when the server sent an object
-  constructor(status: number, detail?: string, detailData?: any) {
+  detailData?: unknown; // structured detail ({ code, reason, ... }) when the server sent an object
+  constructor(status: number, detail?: string, detailData?: unknown) {
     super(detail || `HTTP ${status}`);
     this.status = status;
     this.detail = detail;
@@ -225,20 +225,31 @@ class HttpError extends Error {
 // usually returns a plain string `detail`, but some routes (e.g. download quota/edit) return a
 // structured object with an i18n reason key + numbers so HU/DE users don't get hardcoded English
 // with a dot-decimal separator. Falls back to a generic message for unknown shapes.
-function localizeDetail(d: any): string {
+function localizeDetail(d: unknown): string {
   const t = i18n.t.bind(i18n);
-  if (d?.code === "quota") {
-    if (d.reason === "max_bytes") {
+  // FastAPI request-validation errors: `detail` is an ARRAY of { loc, msg, type }. The UI sends
+  // well-formed requests, so this only fires on genuinely malformed input — surface the offending
+  // field name (localized) rather than FastAPI's raw English `msg`, which reads wrong in a HU UI.
+  if (Array.isArray(d)) {
+    const loc = (d[0] as { loc?: unknown } | undefined)?.loc;
+    const field = Array.isArray(loc)
+      ? loc.filter((s): s is string => typeof s === "string" && s !== "body").pop()
+      : undefined;
+    return field ? t("errors.validation", { field }) : t("errors.generic");
+  }
+  const o = d as { code?: string; reason?: string; limit?: number } | null | undefined;
+  if (o?.code === "quota") {
+    if (o.reason === "max_bytes") {
       const gb = new Intl.NumberFormat(i18n.language, { maximumFractionDigits: 1 }).format(
-        (d.limit || 0) / 1_073_741_824,
+        (o.limit || 0) / 1_073_741_824,
       );
       return t("downloads.errors.quota.max_bytes", { size: gb });
     }
-    if (d.reason === "max_jobs") return t("downloads.errors.quota.max_jobs", { limit: d.limit });
+    if (o.reason === "max_jobs") return t("downloads.errors.quota.max_jobs", { limit: o.limit });
     return t("downloads.errors.quota.generic");
   }
-  if (d?.code === "edit")
-    return t(`downloads.errors.edit.${d.reason}`, t("downloads.errors.edit.generic"));
+  if (o?.code === "edit")
+    return t(`downloads.errors.edit.${o.reason}`, t("downloads.errors.edit.generic"));
   return t("errors.generic");
 }
 
@@ -341,7 +352,11 @@ export function clearActiveAccount(): void {
   }
 }
 
-async function req(url: string, opts: RequestInit = {}, cfg: ReqConfig = {}): Promise<any> {
+async function req<T = unknown>(
+  url: string,
+  opts: RequestInit = {},
+  cfg: ReqConfig = {},
+): Promise<T> {
   const method = opts.method ?? "GET";
   const canRetry = cfg.idempotent ?? method === "GET";
   let attempt = 0;
@@ -386,7 +401,7 @@ async function req(url: string, opts: RequestInit = {}, cfg: ReqConfig = {}): Pr
     if (!r.ok) {
       // Capture the server's reason (FastAPI returns `{ detail }`) so callers can react.
       let detail: string | undefined;
-      let detailData: any;
+      let detailData: unknown;
       try {
         const body = await r.json();
         if (body && typeof body.detail === "string") detail = body.detail;
@@ -417,7 +432,9 @@ async function req(url: string, opts: RequestInit = {}, cfg: ReqConfig = {}): Pr
       }
       throw new HttpError(r.status, detail, detailData);
     }
-    return r.status === 204 ? null : r.json();
+    // The single audited cast in this layer: the server contract is asserted here, once, so
+    // every typed api.* method (T comes from its declared return type) is checked from here up.
+    return (r.status === 204 ? null : await r.json()) as T;
   }
 }
 
@@ -1098,7 +1115,8 @@ export const api = {
   // (the endpoint answers `{authenticated:false}` rather than 401, so the public landing never
   // logs a failed request to the console). React Query treats the null as data, not an error.
   me: async (): Promise<Me | null> => {
-    const r = await req("/api/me");
+    // 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;
   },
   accounts: (): Promise<Account[]> => req("/api/me/accounts"),
@@ -1176,7 +1194,8 @@ export const api = {
   ) => req(`/api/channels/${id}`, { method: "PATCH", body: JSON.stringify(patch) }),
   resetChannelBackfill: (id: string) =>
     req(`/api/channels/${id}/reset-backfill`, { method: "POST" }),
-  deepAll: (on = true) => req(`/api/sync/deep-all?on=${on}`, { method: "POST" }),
+  deepAll: (on = true): Promise<{ updated: number; deep_requested: boolean }> =>
+    req(`/api/sync/deep-all?on=${on}`, { method: "POST" }),
   attachChannelTag: (id: string, tagId: number) =>
     req(`/api/channels/${id}/tags`, { method: "POST", body: JSON.stringify({ tag_id: tagId }) }),
   detachChannelTag: (id: string, tagId: number) =>
@@ -1184,7 +1203,15 @@ export const api = {
   unsubscribeChannel: (id: string) => req(`/api/channels/${id}/subscription`, { method: "DELETE" }),
   discoveredChannels: (): Promise<DiscoveredChannel[]> => req("/api/channels/discovery"),
   subscribeChannel: (id: string) => req(`/api/channels/${id}/subscribe`, { method: "POST" }),
-  syncSubscriptions: () => req("/api/sync/subscriptions", { method: "POST" }),
+  syncSubscriptions: (): Promise<{
+    subscriptions: number;
+    channels_new: number;
+    channels_detailed: number;
+    removed_stale: number;
+    prune_skipped: boolean;
+    quota_used_estimate: number;
+    quota_remaining_today: number;
+  }> => req("/api/sync/subscriptions", { method: "POST" }),
   // --- channel page (About detail + ephemeral exploration of un-subscribed channels) ---
   channelDetail: (id: string): Promise<ChannelDetail> => req(`/api/channels/${id}`),
   // Open an un-subscribed channel for browsing: ingest a page of its uploads (newest-first,