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/src/model_library/paths.rs

@@ -3,6 +3,18 @@ use std::path::{Path, PathBuf};
 const MODEL_LIBRARY_DIR: &str = "model-library";
 const INDEX_FILE: &str = "index.json";
 const MANAGED_MODELS_DIR: &str = "managed-models";
+const DEFAULT_MODELS_DIR: &str = "models";
+
+/// Resolves the models directory from the user-configured `models_dir`.
+/// - `None`        → `./models` (relative to the app's working directory)
+/// - `Some("")`    → `./models`
+/// - `Some(path)`  → that exact path
+pub fn resolve_models_dir(models_dir: Option<&str>) -> PathBuf {
+    match models_dir {
+        None | Some("") => PathBuf::from(DEFAULT_MODELS_DIR),
+        Some(path) => PathBuf::from(path),
+    }
+}
 
 pub fn model_library_dir(app_data_dir: &Path) -> PathBuf {
     app_data_dir.join(MODEL_LIBRARY_DIR)
@@ -12,6 +24,9 @@ pub fn model_library_index_path(app_data_dir: &Path) -> PathBuf {
     model_library_dir(app_data_dir).join(INDEX_FILE)
 }
 
+/// Legacy path for the managed-models location inside AppData.
+/// Used only for index persistence — downloads now go to the user-configured
+/// `models_dir` (defaults to `./models` in the project root).
 pub fn managed_models_dir(app_data_dir: &Path) -> PathBuf {
     model_library_dir(app_data_dir).join(MANAGED_MODELS_DIR)
 }