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/store/chat/streamStore.ts

@@ -24,7 +24,6 @@ import { getSkillDiscovery } from '../../lib/skill-discovery';
 import { useOfflineStore, isOffline } from '../../store/offlineStore';
 import { useConnectionStore } from '../../store/connectionStore';
 import { createLogger } from '../../lib/logger';
-import { speechSynth } from '../../lib/speech-synth';
 import { generateRandomString } from '../../lib/crypto-utils';
 import type { ChatModeType, ChatModeConfig, Subtask } from '../../components/ai';
 import type { ToolCallStep } from '../../components/ai';
@@ -440,22 +439,6 @@ export const useStreamStore = create<StreamState>()(
             };
             _chat?.updateMessages(msgs => [...msgs, handMsg]);
 
-            if (name === 'speech' && status === 'completed' && result && typeof result === 'object') {
-              const res = result as Record<string, unknown>;
-              if (res.tts_method === 'browser' && typeof res.text === 'string' && res.text) {
-                speechSynth.speak({
-                  text: res.text as string,
-                  voice: (res.voice as string) || undefined,
-                  language: (res.language as string) || undefined,
-                  rate: typeof res.rate === 'number' ? res.rate : undefined,
-                  pitch: typeof res.pitch === 'number' ? res.pitch : undefined,
-                  volume: typeof res.volume === 'number' ? res.volume : undefined,
-                }).catch((err: unknown) => {
-                  const logger = createLogger('speech-synth');
-                  logger.warn('Browser TTS failed', { error: String(err) });
-                });
-              }
-            }
           },
           onSubtaskStatus: (taskId: string, description: string, status: string, detail?: string) => {
             // Map backend status to frontend Subtask status