feat: Tier-1 Model Manager with Hot-Swapping, HF Download Engine, bug fixes, and test repairs

- Phase 4: Hugging Face Download Engine — streaming GGUF downloads with progress, cancellation, and managed-model storage
- Phase 5: Model Source Validation — source-mode compatibility, path/file validation, inline error UI with recovery guidance
- Hot-swap: stop runtime → update model_path → restart atomic command
- Model library: folder open, delete model file, local model management
- SettingsPage unified hub: local models list + HF search with 3-state action buttons
- Bug fixes: send button text restored (4 failing tests), reasoning-toggle aria-label, Rust warnings
- All 89 tests passing, cargo check clean

tauri-app/src-tauri/Cargo.toml

@@ -25,6 +25,7 @@ tauri-plugin-dialog = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 reqwest = { version = "0.12", default-features = false, features = ["json", "rustls-tls"] }
+tokio = { version = "1", features = ["fs", "io-util", "sync", "time"] }
 runtime-deps = { path = "../../crates/runtime-deps" }
 
 [profile.release]