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/constants/hands.ts

@@ -16,11 +16,7 @@ export const HAND_IDS = {
   TRADER: 'trader',
   CLIP: 'clip',
   TWITTER: 'twitter',
-  // Additional hands from backend
-  SLIDESHOW: 'slideshow',
-  SPEECH: 'speech',
   QUIZ: 'quiz',
-  WHITEBOARD: 'whiteboard',
 } as const;
 
 export type HandIdType = typeof HAND_IDS[keyof typeof HAND_IDS];
@@ -49,10 +45,7 @@ export const HAND_CATEGORY_MAP: Record<string, HandCategoryType> = {
   [HAND_IDS.LEAD]: HAND_CATEGORIES.COMMUNICATION,
   [HAND_IDS.TWITTER]: HAND_CATEGORIES.COMMUNICATION,
   [HAND_IDS.CLIP]: HAND_CATEGORIES.CONTENT,
-  [HAND_IDS.SLIDESHOW]: HAND_CATEGORIES.CONTENT,
-  [HAND_IDS.SPEECH]: HAND_CATEGORIES.CONTENT,
   [HAND_IDS.QUIZ]: HAND_CATEGORIES.PRODUCTIVITY,
-  [HAND_IDS.WHITEBOARD]: HAND_CATEGORIES.PRODUCTIVITY,
 };
 
 // === Helper Functions ===