Extract casting functionality (Chromecast, AirPlay, DLNA) to standalone qbz-cast crate. This crate is Tauri-agnostic and can be used by TUI/CLI clients. - Move device connection, discovery, threading, and media server to crate - Keep only Tauri commands in src-tauri/src/cast/ - Thin re-exports in src-tauri for backwards compatibility - Remove duplicate dependencies from src-tauri Cargo.toml