Make the app work better on iPhone and iPad

- Text fields are at least 16px on touch screens (stops iOS zooming on focus)
- Chapter swipe ignores sheets, fields, sliders, sideways-scrolling rows and text selection
- vh -> dvh for sheets, palette and dialogs
- Bigger tap targets for small links, chips and the logo; no double-tap delay
- No tap flash or landscape text inflation

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

src/components/ShortcutsHelp.jsx

@@ -53,7 +53,7 @@ export default function ShortcutsHelp({ onClose }) {
 
   return (
     <div style={themeVars(readerTheme)} className="fixed inset-0 z-[90] flex items-center justify-center bg-black/40 px-3 font-sans" onMouseDown={(e) => { if (e.target === e.currentTarget) onClose(); }}>
-      <div role="dialog" aria-modal="true" aria-label="Keyboard shortcuts" className="max-h-[80vh] w-full max-w-sm overflow-y-auto rounded-2xl border border-[var(--r-line)] bg-[var(--r-bar)] p-5 text-[var(--r-text)] shadow-2xl">
+      <div role="dialog" aria-modal="true" aria-label="Keyboard shortcuts" className="max-h-[80dvh] w-full max-w-sm overflow-y-auto rounded-2xl border border-[var(--r-line)] bg-[var(--r-bar)] p-5 text-[var(--r-text)] shadow-2xl">
         <div className="mb-3 flex items-center justify-between">
           <h2 className="text-base font-medium">Keyboard shortcuts</h2>
           <button ref={closeRef} type="button" onClick={onClose} className="rounded-lg px-2 py-1 text-sm text-[var(--r-sub)] hover:bg-black/5">Close</button>