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/components/ChatView.tsx

@@ -667,7 +667,7 @@ export default function ChatView({ db, chatId, server, apiKey, model, onChatCrea
                     type="button"
                     aria-label="Send message"
                   >
-                    <ArrowUp size={18} weight="duotone" />
+                    Send <ArrowUp size={18} weight="duotone" />
                   </button>
                 )}
               </div>