refactor(hands): 移除空壳 Hand — Whiteboard/Slideshow/Speech (Phase 5)

删除 3 个仅含 UI 占位的 Hand，清理 Rust 实现与前端引用：
- Rust: whiteboard.rs(422行) + slideshow.rs(797行) + speech.rs(442行)
- 前端: WhiteboardCanvas + SlideshowRenderer + speech-synth + 相关类型/常量
- 配置: 3 个 HAND.toml
- 净减 ~5400 行，Hands 9→6(启用) + Quiz/Browser/Researcher/Collector/Clip/Twitter/Reminder

desktop/src/lib/speech-synth.ts

@@ -1,195 +0,0 @@
-/**
- * Speech Synthesis Service — Browser TTS via Web Speech API
- *
- * Provides text-to-speech playback using the browser's native SpeechSynthesis API.
- * Zero external dependencies, works offline, supports Chinese and English voices.
- *
- * Architecture:
- * - SpeechHand (Rust) returns tts_method + text + voice config
- * - This service handles Browser TTS playback in the webview
- * - OpenAI/Azure TTS is handled via backend API calls
- */
-
-export interface SpeechSynthOptions {
-  text: string;
-  voice?: string;
-  language?: string;
-  rate?: number;
-  pitch?: number;
-  volume?: number;
-}
-
-export interface SpeechSynthState {
-  playing: boolean;
-  paused: boolean;
-  currentText: string | null;
-  voices: SpeechSynthesisVoice[];
-}
-
-type SpeechEventCallback = (state: SpeechSynthState) => void;
-
-class SpeechSynthService {
-  private synth: SpeechSynthesis | null = null;
-  private currentUtterance: SpeechSynthesisUtterance | null = null;
-  private listeners: Set<SpeechEventCallback> = new Set();
-  private cachedVoices: SpeechSynthesisVoice[] = [];
-
-  constructor() {
-    if (typeof window !== 'undefined' && window.speechSynthesis) {
-      this.synth = window.speechSynthesis;
-      this.loadVoices();
-      // Voices may load asynchronously
-      this.synth.onvoiceschanged = () => this.loadVoices();
-    }
-  }
-
-  private loadVoices() {
-    if (!this.synth) return;
-    this.cachedVoices = this.synth.getVoices();
-    this.notify();
-  }
-
-  private notify() {
-    const state = this.getState();
-    this.listeners.forEach(cb => cb(state));
-  }
-
-  /** Subscribe to state changes */
-  subscribe(callback: SpeechEventCallback): () => void {
-    this.listeners.add(callback);
-    return () => this.listeners.delete(callback);
-  }
-
-  /** Get current state */
-  getState(): SpeechSynthState {
-    return {
-      playing: this.synth?.speaking ?? false,
-      paused: this.synth?.paused ?? false,
-      currentText: this.currentUtterance?.text ?? null,
-      voices: this.cachedVoices,
-    };
-  }
-
-  /** Check if TTS is available */
-  isAvailable(): boolean {
-    return this.synth != null;
-  }
-
-  /** Get available voices, optionally filtered by language */
-  getVoices(language?: string): SpeechSynthesisVoice[] {
-    if (!language) return this.cachedVoices;
-    const langPrefix = language.split('-')[0].toLowerCase();
-    return this.cachedVoices.filter(v =>
-      v.lang.toLowerCase().startsWith(langPrefix)
-    );
-  }
-
-  /** Speak text with given options */
-  speak(options: SpeechSynthOptions): Promise<void> {
-    return new Promise((resolve, reject) => {
-      if (!this.synth) {
-        reject(new Error('Speech synthesis not available'));
-        return;
-      }
-
-      // Cancel any ongoing speech
-      this.stop();
-
-      const utterance = new SpeechSynthesisUtterance(options.text);
-      this.currentUtterance = utterance;
-
-      // Set language
-      utterance.lang = options.language ?? 'zh-CN';
-
-      // Set voice if specified
-      if (options.voice && options.voice !== 'default') {
-        const voice = this.cachedVoices.find(v =>
-          v.name === options.voice || v.voiceURI === options.voice
-        );
-        if (voice) utterance.voice = voice;
-      } else {
-        // Auto-select best voice for the language
-        this.selectBestVoice(utterance, options.language ?? 'zh-CN');
-      }
-
-      // Set parameters
-      utterance.rate = options.rate ?? 1.0;
-      utterance.pitch = options.pitch ?? 1.0;
-      utterance.volume = options.volume ?? 1.0;
-
-      utterance.onstart = () => {
-        this.notify();
-      };
-
-      utterance.onend = () => {
-        this.currentUtterance = null;
-        this.notify();
-        resolve();
-      };
-
-      utterance.onerror = (event) => {
-        this.currentUtterance = null;
-        this.notify();
-        // "canceled" is not a real error (happens on stop())
-        if (event.error !== 'canceled') {
-          reject(new Error(`Speech error: ${event.error}`));
-        } else {
-          resolve();
-        }
-      };
-
-      this.synth.speak(utterance);
-    });
-  }
-
-  /** Pause current speech */
-  pause() {
-    this.synth?.pause();
-    this.notify();
-  }
-
-  /** Resume paused speech */
-  resume() {
-    this.synth?.resume();
-    this.notify();
-  }
-
-  /** Stop current speech */
-  stop() {
-    this.synth?.cancel();
-    this.currentUtterance = null;
-    this.notify();
-  }
-
-  /** Auto-select the best voice for a language */
-  private selectBestVoice(utterance: SpeechSynthesisUtterance, language: string) {
-    const langPrefix = language.split('-')[0].toLowerCase();
-    const candidates = this.cachedVoices.filter(v =>
-      v.lang.toLowerCase().startsWith(langPrefix)
-    );
-
-    if (candidates.length === 0) return;
-
-    // Prefer voices with "Neural" or "Enhanced" in name (higher quality)
-    const neural = candidates.find(v =>
-      v.name.includes('Neural') || v.name.includes('Enhanced') || v.name.includes('Premium')
-    );
-    if (neural) {
-      utterance.voice = neural;
-      return;
-    }
-
-    // Prefer local voices (work offline)
-    const local = candidates.find(v => v.localService);
-    if (local) {
-      utterance.voice = local;
-      return;
-    }
-
-    // Fall back to first matching voice
-    utterance.voice = candidates[0];
-  }
-}
-
-// Singleton instance
-export const speechSynth = new SpeechSynthService();