Easier navigation on iPhone

- Phones get a bottom tab bar (Studies, Bible, Plans, Prayer, Account); wider screens keep the header nav
- The Account menu opens upward from the bar; toasts sit above it
- Reader and study pages get a labelled Home button instead of a small arrow

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

src/hooks/useIsPhone.js

@@ -0,0 +1,19 @@
+import { useEffect, useState } from 'react';
+
+const QUERY = '(max-width: 639px)';
+const read = () => (typeof window !== 'undefined' && typeof window.matchMedia === 'function' ? window.matchMedia(QUERY).matches : false);
+
+// True on a phone-width screen (under Tailwind's `sm`). False where matchMedia is missing (tests), so the full
+// layout is what renders there.
+export function useIsPhone() {
+  const [phone, setPhone] = useState(read);
+  useEffect(() => {
+    if (typeof window.matchMedia !== 'function') return undefined;
+    const mq = window.matchMedia(QUERY);
+    const update = () => setPhone(mq.matches);
+    update();
+    mq.addEventListener?.('change', update);
+    return () => mq.removeEventListener?.('change', update);
+  }, []);
+  return phone;
+}