Resizable right column in the Study stage; start on the chosen page without a flash

The Study stage's right column has a drag handle (mouse, arrow keys, double-click
or Home to reset); the width is remembered per device. The app now starts on the
user's chosen home page from the first render instead of showing My Studies
first. Tests wait on page headings so a lazily loaded page can't be confused
with hidden text from the previous one.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/hooks/useResizableWidth.js

@@ -0,0 +1,75 @@
+// A drag-to-resize width for a side panel (the Study stage's right column). The handle owns
+// pointer dragging, arrow keys and double-click reset; the width is remembered per device.
+import { useCallback, useRef, useState } from 'react';
+
+const STEP = 24;
+const BIG_STEP = 96;
+
+// containerRef: the element the panel sits at the right edge of (for measuring and clamping).
+export function useResizableWidth({ storageKey, min = 320, maxAbsolute = 760, fallback = 448, reserve = 600, containerRef }) {
+  const maxFor = () => {
+    const w = containerRef.current?.getBoundingClientRect().width ?? 0;
+    return w > 0 ? Math.max(min, Math.min(maxAbsolute, w - reserve)) : maxAbsolute;
+  };
+  const clamp = (n) => Math.round(Math.min(maxFor(), Math.max(min, n)));
+
+  const [width, setWidthState] = useState(() => {
+    try {
+      const n = Number(localStorage.getItem(storageKey));
+      return Number.isFinite(n) && n > 0 ? Math.min(maxAbsolute, Math.max(min, Math.round(n))) : null;
+    } catch { return null; }
+  });
+  const dragRef = useRef(null);
+
+  const commit = useCallback((n) => {
+    setWidthState(n);
+    try {
+      if (n == null) localStorage.removeItem(storageKey);
+      else localStorage.setItem(storageKey, String(n));
+    } catch { /* storage unavailable */ }
+  }, [storageKey]);
+
+  // Width of the panel right now (state, else measured from the panel element, else a default).
+  const current = (panelEl) => width ?? (panelEl?.getBoundingClientRect().width || fallback);
+
+  const handleProps = (panelRef) => ({
+    role: 'separator',
+    'aria-orientation': 'vertical',
+    'aria-label': 'Resize side panel',
+    'aria-valuemin': min,
+    'aria-valuemax': maxAbsolute,
+    'aria-valuenow': Math.round(current(panelRef.current)),
+    tabIndex: 0,
+    onPointerDown: (e) => {
+      if (e.button !== undefined && e.button !== 0) return;
+      e.currentTarget.setPointerCapture?.(e.pointerId);
+      dragRef.current = { id: e.pointerId };
+      document.body.style.userSelect = 'none';
+    },
+    onPointerMove: (e) => {
+      if (!dragRef.current) return;
+      const right = containerRef.current?.getBoundingClientRect().right ?? window.innerWidth;
+      const next = clamp(right - e.clientX);
+      dragRef.current.width = next;
+      containerRef.current?.style.setProperty('--study-right', `${next}px`); // smooth: no React render per move
+    },
+    onPointerUp: (e) => {
+      if (!dragRef.current) return;
+      e.currentTarget.releasePointerCapture?.(e.pointerId);
+      const done = dragRef.current.width;
+      dragRef.current = null;
+      document.body.style.userSelect = '';
+      if (done != null) commit(done);
+    },
+    onKeyDown: (e) => {
+      const step = e.shiftKey ? BIG_STEP : STEP;
+      if (e.key === 'ArrowLeft') { e.preventDefault(); commit(clamp(current(panelRef.current) + step)); }
+      else if (e.key === 'ArrowRight') { e.preventDefault(); commit(clamp(current(panelRef.current) - step)); }
+      else if (e.key === 'Home' || e.key === 'Escape') { e.preventDefault(); containerRef.current?.style.removeProperty('--study-right'); commit(null); }
+    },
+    onDoubleClick: () => { containerRef.current?.style.removeProperty('--study-right'); commit(null); },
+    title: 'Drag to resize · double-click to reset',
+  });
+
+  return { width, handleProps };
+}