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

crates/zclaw-hands/src/hands/mod.rs

@@ -1,9 +1,6 @@
 //! Educational Hands - Teaching and presentation capabilities
 //!
-//! This module provides hands for interactive classroom experiences:
-//! - Whiteboard: Drawing and annotation
-//! - Slideshow: Presentation control
-//! - Speech: Text-to-speech synthesis
+//! This module provides hands for interactive experiences:
 //! - Quiz: Assessment and evaluation
 //! - Browser: Web automation
 //! - Researcher: Deep research and analysis
@@ -11,9 +8,6 @@
 //! - Clip: Video processing
 //! - Twitter: Social media automation
 
-mod whiteboard;
-mod slideshow;
-mod speech;
 pub mod quiz;
 mod browser;
 mod researcher;
@@ -22,9 +16,6 @@ mod clip;
 mod twitter;
 pub mod reminder;
 
-pub use whiteboard::*;
-pub use slideshow::*;
-pub use speech::*;
 pub use quiz::*;
 pub use browser::*;
 pub use researcher::*;