refactor(api): extract 8 content domains into api/*.ts slices (R7.5 S1)

Move feed, channels, sync, playlists, tags, notifications, saved-views and
quota out of the lib/api.ts god object into per-domain slice modules, each
owning its own types + method slice and spread into the façade. The `api`
object shape and all 54 call sites are unchanged; account/auth/setup/admin
stay inline until S2.

Reunites the interleaved tags read/CRUD that motivated the epic.

frontend/src/lib/api/quota.ts

@@ -0,0 +1,29 @@
+// YouTube API quota usage: the signed-in user's own totals and the admin cross-user view.
+// Only touches core's req.
+import { req } from "./core";
+
+export interface MyUsage {
+  today: number;
+  last_7d: number;
+  last_30d: number;
+  all_time: number;
+  by_action: Record<string, number>;
+}
+
+export interface AdminQuotaRow {
+  user_id: number | null;
+  email: string;
+  total: number;
+  by_action: Record<string, number>;
+}
+
+export interface AdminQuota {
+  range_days: number;
+  rows: AdminQuotaRow[];
+  daily: { day: string; total: number }[];
+}
+
+export const quotaApi = {
+  myUsage: (): Promise<MyUsage> => req("/api/quota/my-usage"),
+  adminQuota: (days = 30): Promise<AdminQuota> => req(`/api/quota/admin?days=${days}`),
+};