feat: enhance AppConfig loading to merge default providers and set API key

src/model/app_config.rs

@@ -60,9 +60,15 @@ impl AppConfig {
     pub fn load() -> Self {
         let store = super::store::Store::new();
         let path = store.base_dir.join("app_config.json");
-        std::fs::read_to_string(path)
+        let mut cfg: AppConfig = std::fs::read_to_string(path)
             .ok()
             .and_then(|s| serde_json::from_str(&s).ok())
-            .unwrap_or_default()
+            .unwrap_or_default();
+        // Merge any default providers not present in the loaded config
+        let defaults = Self::default();
+        for (name, provider) in defaults.providers {
+            cfg.providers.entry(name).or_insert(provider);
+        }
+        cfg
     }
 }