feat: initial implementation of Focus RSVP speed reader

- Pure frontend SPA (Vite + React 19 + TypeScript + Tailwind CSS v4)
- Spritz ORP engine with absolute center crosshair gaze fixation
- Multilingual hybrid segmentation via Intl.Segmenter with punctuation attachment
- Adaptive breathing rhythm delay algorithm for natural comprehension
- IndexedDB local bookshelf, progress persistence, and client-side EPUB/TXT/MD parsing
- Curated sci-fi, philosophy, and classic speech samples
- Dark/OLED/Sepia/Slate/Nord themes and rich keyboard controls

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

src/components/ShortcutsModal.tsx

@@ -0,0 +1,65 @@
+import { HelpCircle, X } from 'lucide-react';
+import React from 'react';
+
+interface ShortcutsModalProps {
+  isOpen: boolean;
+  onClose: () => void;
+}
+
+const SHORTCUTS = [
+  { key: '空格键 (Space)', action: '播放 / 暂停' },
+  { key: '← / →', action: '快退 / 快进 10 个词' },
+  { key: 'Shift + ← / →', action: '跳转到 上一句 / 下一句' },
+  { key: '↑ / ↓', action: '微调阅读语速 ±25 WPM' },
+  { key: 'Shift + ↑ / ↓', action: '加速调节语速 ±50 WPM' },
+  { key: '[ / ] 或 PageUp/Down', action: '跳转到 上一章 / 下一章' },
+  { key: 'R', action: '从头重新开始阅读' },
+  { key: 'F', action: '切换全屏专注模式' },
+  { key: 'Esc', action: '关闭当前浮层 / 弹窗' },
+];
+
+export const ShortcutsModal: React.FC<ShortcutsModalProps> = ({
+  isOpen,
+  onClose,
+}) => {
+  if (!isOpen) return null;
+
+  return (
+    <div className="fixed inset-0 z-50 flex items-center justify-center p-4 bg-black/60 backdrop-blur-sm animate-fade-in select-none">
+      <div className="relative w-full max-w-md bg-white dark:bg-neutral-900 border border-neutral-200 dark:border-neutral-800 rounded-2xl shadow-2xl flex flex-col max-h-[85vh] overflow-hidden">
+        {/* Header */}
+        <div className="flex items-center justify-between px-6 py-4 border-b border-neutral-200 dark:border-neutral-800">
+          <div className="flex items-center space-x-2">
+            <HelpCircle className="w-5 h-5 text-rose-500" />
+            <h2 className="text-lg font-bold text-neutral-900 dark:text-neutral-100">
+              键盘极速快捷键
+            </h2>
+          </div>
+          <button
+            onClick={onClose}
+            className="p-1 rounded-lg text-neutral-400 hover:text-neutral-900 dark:hover:text-white hover:bg-neutral-100 dark:hover:bg-neutral-800 transition-colors"
+          >
+            <X className="w-5 h-5" />
+          </button>
+        </div>
+
+        {/* Shortcuts list */}
+        <div className="p-6 space-y-2.5 flex-1 overflow-y-auto">
+          {SHORTCUTS.map((s, idx) => (
+            <div
+              key={idx}
+              className="flex items-center justify-between py-1.5 border-b border-neutral-100 dark:border-neutral-800/60 last:border-0"
+            >
+              <kbd className="px-2.5 py-1 text-xs font-mono font-semibold bg-neutral-100 dark:bg-neutral-800 text-neutral-800 dark:text-neutral-200 rounded-md border border-neutral-200 dark:border-neutral-700 shadow-sm">
+                {s.key}
+              </kbd>
+              <span className="text-sm text-neutral-600 dark:text-neutral-300">
+                {s.action}
+              </span>
+            </div>
+          ))}
+        </div>
+      </div>
+    </div>
+  );
+};