refactor(header): move --hdr-h measurement into PageShell (S7 review)

Addresses the S7 mobile-fix code-review findings:

- (altitude) The header-height→content-pad contract lived across two files with
  an implicit DOM coupling: Header measured itself and wrote --hdr-h onto
  `el.parentElement`, assuming PageShell rendered it as a direct child of the
  padded column. Move the whole contract into PageShell, which owns BOTH the
  header slot and the padded content wrapper: Header is now forwardRef and
  PageShell attaches a ref (via cloneElement), measures the header with a
  ResizeObserver, and writes --hdr-h straight onto the content wrapper that
  consumes it — no parent assumption, producer and consumer together.
- (simplification) The `+ 8` gap is now documented as the header's own gap-y-2
  row gap reused below it, not a bare magic number.
- (test-coverage) PageShell.dom.test.tsx: stub ResizeObserver + the header's
  offset metrics and assert --hdr-h is published onto the content wrapper (and
  absent with no header).

Re-verified live at 440 (mobile, no overlap) and desktop (one row). FE gate:
tsc / knip / lint clean, 220 vitest pass.

frontend/src/components/Header.tsx

@@ -1,4 +1,4 @@
-import { useLayoutEffect, useRef, type ReactNode } from "react";
+import { forwardRef, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import { Youtube } from "lucide-react";
 import type { Me } from "../lib/api";
@@ -16,15 +16,18 @@ import { useChannels, useChannelsActions } from "./ChannelsProvider";
 // The floating top header: a row of glass pills over the content — a ModuleName pill (label +
 // accent dot + back/forward arrows), the module's SearchBar where it has one, and the per-user
 // sync chip at the right end.
-export default function Header({
+// `forwardRef` so PageShell can attach a ref to the header's root box and measure its real (possibly
-  me,
+// wrapped) height — that measurement + the `--hdr-h` content pad it drives live in PageShell, which
-  onYtSearch,
+// owns both the header slot and the padded content wrapper (E5 S7).
-}: {
+const Header = forwardRef<
+  HTMLDivElement,
+  {
     me: Me;
     // Trigger a live YouTube search for the current term (feed only; hidden for the demo account,
     // which can't spend the shared quota).
     onYtSearch: (q: string) => void;
-}) {
+  }
+>(function Header({ me, onYtSearch }, ref) {
   const { t } = useTranslation();
   const { page } = useNavigation();
   const { setPage } = useNavigationActions();
@@ -46,27 +49,6 @@ export default function Header({
   // Drop input focus on Go/Enter for the live-filter modules (feed's Go escalates to YouTube).
   const blur = () => (document.activeElement as HTMLElement | null)?.blur?.();
 
-  // The header floats (absolute) over the content, which pads its top by `--hdr-h`. That default is
-  // sized for a single row; on a narrow window (esp. mobile) the flex-wrap header grows to 2–3 rows,
-  // so a FIXED pad would let the content toolbar (sort / count / filter chips) slide up under it.
-  // Measure the real header box and publish its height as `--hdr-h` on the content column, so the pad
-  // always tracks the actual (wrapped) header height at any width.
-  const headerRef = useRef<HTMLDivElement>(null);
-  useLayoutEffect(() => {
-    const el = headerRef.current;
-    const host = el?.parentElement; // the `relative` content column (PageShell renders a fragment)
-    if (!el || !host) return;
-    const apply = () =>
-      host.style.setProperty("--hdr-h", `${el.offsetTop + el.offsetHeight + 8}px`);
-    apply();
-    const ro = new ResizeObserver(apply);
-    ro.observe(el);
-    return () => {
-      ro.disconnect();
-      host.style.removeProperty("--hdr-h");
-    };
-  }, []);
-
   // Per-page SearchBar wiring — null for modules without a search.
   let search: ReactNode = null;
   if (page === "feed") {
@@ -116,7 +98,7 @@ export default function Header({
     // actual left zone: it sits just inside the content, reclaiming the space when the nav rail is
     // slim and/or no filter sidebar is present — no hard-coded offset.
     <div
-      ref={headerRef}
+      ref={ref}
       className="pointer-events-none absolute top-3 left-3 right-[10%] z-chrome flex flex-wrap items-center gap-x-3 gap-y-2"
     >
       <ModuleName
@@ -140,4 +122,6 @@ export default function Header({
       )}
     </div>
   );
-}
+});
+
+export default Header;

frontend/src/components/PageShell.dom.test.tsx

@@ -0,0 +1,53 @@
+// @vitest-environment jsdom
+//
+// E5 S7: PageShell measures the floating header and publishes its real height as `--hdr-h` onto the
+// content wrapper, so a header that flex-wraps to 2–3 rows on a narrow window never overlaps the
+// content below it. jsdom does no layout (offsetHeight is 0) and has no ResizeObserver, so we stub
+// both: a ResizeObserver that hands us its callback, and offset metrics on the measured header.
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { render, cleanup } from "@testing-library/react";
+import PageShell from "./PageShell";
+
+vi.mock("./PageScroller", () => ({
+  default: ({ children }: { children: React.ReactNode }) => <div>{children}</div>,
+}));
+
+let roCb: (() => void) | null = null;
+beforeEach(() => {
+  roCb = null;
+  vi.stubGlobal(
+    "ResizeObserver",
+    class {
+      constructor(cb: () => void) {
+        roCb = cb;
+      }
+      observe() {}
+      disconnect() {}
+    },
+  );
+});
+afterEach(() => {
+  cleanup();
+  vi.unstubAllGlobals();
+});
+
+describe("PageShell --hdr-h (E5 S7)", () => {
+  it("publishes the measured header height onto the content wrapper", () => {
+    const { container } = render(
+      <PageShell header={<div data-testid="hdr">H</div>}>content</PageShell>,
+    );
+    const content = container.querySelector('[class*="pt-(--hdr-h)"]') as HTMLElement;
+    expect(content).toBeTruthy();
+    // Give the header a real (multi-row) box, then fire the ResizeObserver callback the effect made.
+    const hdr = container.querySelector('[data-testid="hdr"]') as HTMLElement;
+    Object.defineProperty(hdr, "offsetTop", { value: 12, configurable: true });
+    Object.defineProperty(hdr, "offsetHeight", { value: 96, configurable: true });
+    roCb?.();
+    expect(content.style.getPropertyValue("--hdr-h")).toBe("116px"); // 12 + 96 + 8
+  });
+
+  it("adds no `--hdr-h` pad when there is no header", () => {
+    const { container } = render(<PageShell>content</PageShell>);
+    expect(container.querySelector('[class*="pt-(--hdr-h)"]')).toBeNull();
+  });
+});

frontend/src/components/PageShell.tsx

@@ -1,4 +1,15 @@
-import { createContext, useContext, useState, type ReactNode } from "react";
+import {
+  cloneElement,
+  createContext,
+  isValidElement,
+  useContext,
+  useLayoutEffect,
+  useRef,
+  useState,
+  type ReactElement,
+  type ReactNode,
+  type Ref,
+} from "react";
 import { createPortal } from "react-dom";
 import PageScroller from "./PageScroller";
 
@@ -52,10 +63,39 @@ export default function PageShell({
   fadeTop?: boolean;
 }) {
   const [toolbarSlot, setToolbarSlot] = useState<HTMLDivElement | null>(null);
+  // The floating header pads the content below it by `--hdr-h`. That must equal the header's REAL
+  // height, which grows as the flex-wrap header wraps to 2–3 rows on a narrow window — a fixed pad
+  // would let the content toolbar slide up under it (E5 S7). Measure the header (a ref forwarded onto
+  // its root box) and write its height straight onto the content wrapper that consumes `--hdr-h`, so
+  // the two live together with no assumption about the header's DOM parent.
+  const headerRef = useRef<HTMLDivElement | null>(null);
+  const contentRef = useRef<HTMLDivElement | null>(null);
+  useLayoutEffect(() => {
+    const h = headerRef.current;
+    const c = contentRef.current;
+    if (!h || !c) return;
+    // `+ 8` = the header's own gap-y-2 row gap (0.5rem), reused as the gap below it.
+    const apply = () => c.style.setProperty("--hdr-h", `${h.offsetTop + h.offsetHeight + 8}px`);
+    apply();
+    const ro = new ResizeObserver(apply);
+    ro.observe(h);
+    return () => {
+      ro.disconnect();
+      c.style.removeProperty("--hdr-h");
+    };
+  }, []);
+  const measuredHeader = isValidElement(header)
+    ? cloneElement(header as ReactElement<{ ref?: Ref<HTMLDivElement> }>, {
+        ref: headerRef,
+      })
+    : header;
   return (
     <PageToolbarSlot.Provider value={toolbarSlot}>
-      {header}
+      {measuredHeader}
-      <div className={`flex-1 min-w-0 min-h-0 flex flex-col ${header ? "pt-(--hdr-h)" : ""}`}>
+      <div
+        ref={contentRef}
+        className={`flex-1 min-w-0 min-h-0 flex flex-col ${header ? "pt-(--hdr-h)" : ""}`}
+      >
         {banners}
         <div ref={setToolbarSlot} className="shrink-0" />
         <PageScroller scrollKey={scrollKey} fadeTop={fadeTop}>