refactor(api): honest FeedResponse; derive pageTitleKey from the registry (R7 S3a·2-3)

- FeedResponse claimed a required `limit`, but /api/search/youtube never
  returns it (only /api/feed does) and nothing in the app reads it. Drop the
  field so searchYoutube's Promise<FeedResponse> stops lying; documented for
  re-add if a consumer appears.
- pageMeta's hand-written 13-case Page->title switch now derives from the
  ModuleDef registry via a new optional `titleKey` (the longer tab title, e.g.
  "Channel manager"; falls back to the short nav labelKey). A new page is one
  touch (add it to MODULES) instead of also editing a switch. A test pins all
  13 title keys byte-identical to the old switch (suite 74).

frontend/src/lib/api.ts

@@ -123,10 +123,12 @@ export interface FeedResponse {
   has_more: boolean;
   // Opaque keyset cursor for the next page, or null when this is the last page.
   next_cursor: string | null;
-  limit: number;
   // Only set by the live YouTube search: "scrape" (InnerTube, no API quota) or "api"
   // (search.list, 100 units/page) — lets the UI drop the quota warning in scrape mode.
   source?: "scrape" | "api";
+  // NOTE: /api/feed also returns an echoed `limit`, but /api/search/youtube does not and nothing
+  // in the app reads it, so it's deliberately omitted rather than typed required (a lie for search)
+  // or optional (dead surface). Re-add if a consumer ever needs it.
 }
 
 export type PlaylistKind = "user" | "watch_later";

frontend/src/lib/modules.ts

@@ -26,6 +26,9 @@ export type ModuleDef = {
   id: Page;
   // i18n key for the SHORT display name — the same label the nav rail and the header pill show.
   labelKey: string;
+  // i18n key for the LONGER, descriptive browser-tab title (e.g. "Channel manager" vs the rail's
+  // "Channels"). Absent → the tab title falls back to `labelKey`. Read via `pageTitleKey`.
+  titleKey?: string;
   icon: LucideIcon;
   // Admin/system modules render in their own section (below a divider) in the nav rail.
   system?: boolean;
@@ -44,7 +47,12 @@ const notDemo = (me: Me) => !me.is_demo;
 // Ordered in nav-rail order — this array IS the sequence the rail and the stepper walk.
 const MODULES: readonly ModuleDef[] = [
   { id: "feed", labelKey: "header.account.feed", icon: Home },
-  { id: "channels", labelKey: "header.account.channels", icon: Tv },
+  {
+    id: "channels",
+    labelKey: "header.account.channels",
+    titleKey: "header.channelManager",
+    icon: Tv,
+  },
   { id: "playlists", labelKey: "header.account.playlists", icon: ListVideo },
   { id: "plex", labelKey: "plex.navLabel", icon: Clapperboard, gate: (me) => me.plex_enabled },
   { id: "notifications", labelKey: "inbox.navLabel", icon: Bell, hasBadge: true },
@@ -62,10 +70,11 @@ const MODULES: readonly ModuleDef[] = [
     hasBadge: true,
     gate: notDemo,
   },
-  { id: "stats", labelKey: "header.account.stats", icon: BarChart3 },
+  { id: "stats", labelKey: "header.account.stats", titleKey: "header.usageStats", icon: BarChart3 },
   {
     id: "scheduler",
     labelKey: "header.account.scheduler",
+    titleKey: "header.scheduler",
     icon: Activity,
     system: true,
     gate: isAdmin,
@@ -73,12 +82,27 @@ const MODULES: readonly ModuleDef[] = [
   {
     id: "config",
     labelKey: "header.account.config",
+    titleKey: "header.configuration",
     icon: SlidersHorizontal,
     system: true,
     gate: isAdmin,
   },
-  { id: "users", labelKey: "header.account.users", icon: Users, system: true, gate: isAdmin },
-  { id: "audit", labelKey: "header.account.audit", icon: ScrollText, system: true, gate: isAdmin },
+  {
+    id: "users",
+    labelKey: "header.account.users",
+    titleKey: "header.users",
+    icon: Users,
+    system: true,
+    gate: isAdmin,
+  },
+  {
+    id: "audit",
+    labelKey: "header.account.audit",
+    titleKey: "audit.title",
+    icon: ScrollText,
+    system: true,
+    gate: isAdmin,
+  },
 ];
 
 // Settings is reachable (its own rail button + the ◀/▶ wrap target) but is not a primary module in
@@ -86,6 +110,7 @@ const MODULES: readonly ModuleDef[] = [
 const SETTINGS_MODULE: ModuleDef = {
   id: "settings",
   labelKey: "header.account.settings",
+  titleKey: "settings.title",
   icon: Settings,
 };
 

frontend/src/lib/pageMeta.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, it } from "vitest";
+import { pageTitleKey } from "./pageMeta";
+import type { Page } from "./urlState";
+
+// pageTitleKey now derives from the ModuleDef registry (titleKey ?? labelKey). These are the exact
+// browser-tab title keys the old hand-written switch produced — pin every page so a future labelKey
+// rename (or a dropped titleKey) can't silently change a tab title.
+const EXPECTED: Record<Page, string> = {
+  feed: "header.account.feed",
+  channels: "header.channelManager",
+  playlists: "header.account.playlists",
+  notifications: "inbox.navLabel",
+  messages: "messages.navLabel",
+  downloads: "downloads.navLabel",
+  stats: "header.usageStats",
+  plex: "plex.navLabel",
+  scheduler: "header.scheduler",
+  config: "header.configuration",
+  users: "header.users",
+  audit: "audit.title",
+  settings: "settings.title",
+};
+
+describe("pageTitleKey", () => {
+  it("returns the exact title key for every page (registry-derived)", () => {
+    for (const [page, key] of Object.entries(EXPECTED)) {
+      expect(pageTitleKey(page as Page)).toBe(key);
+    }
+  });
+});

frontend/src/lib/pageMeta.ts

@@ -1,36 +1,12 @@
 import type { Page } from "./urlState";
+import { moduleDef } from "./modules";
 
-// The i18n key for a page's human title — shared by the top-bar header and the browser tab title
-// so the two never drift. Keep in sync with the nav modules in NavSidebar / App's page switch.
+// The i18n key for a page's human title — shared by the top-bar header and the browser tab title so
+// the two never drift. Derived from the single ModuleDef registry (modules.ts): a module's optional
+// `titleKey` is the longer, descriptive tab title (e.g. "Channel manager"), falling back to its
+// short nav `labelKey` when the two are the same. Registering a new Page is then one touch (add it
+// to MODULES) instead of also editing a switch here.
 export function pageTitleKey(page: Page): string {
-  switch (page) {
-    case "feed":
-      return "header.account.feed";
-    case "channels":
-      return "header.channelManager";
-    case "playlists":
-      return "header.account.playlists";
-    case "notifications":
-      return "inbox.navLabel";
-    case "messages":
-      return "messages.navLabel";
-    case "downloads":
-      return "downloads.navLabel";
-    case "stats":
-      return "header.usageStats";
-    case "plex":
-      return "plex.navLabel";
-    case "scheduler":
-      return "header.scheduler";
-    case "config":
-      return "header.configuration";
-    case "users":
-      return "header.users";
-    case "audit":
-      return "audit.title";
-    case "settings":
-      return "settings.title";
-    default:
-      return "header.account.feed";
-  }
+  const def = moduleDef(page);
+  return def.titleKey ?? def.labelKey;
 }