[bugfix]: Check for Navidrome authentication on startup

Resolves #403.
This PR introduces a startup check for Navidrome that tries a simple API request (/songs) before loading homepage.
If the check fails, Navidrome API will fallback to trying saved password (if available).

Notes:
- It might also be worthwhile to do a periodic poll?

src/renderer/types.ts

@@ -213,3 +213,9 @@ export enum FontType {
 }
 
 export type TitleTheme = 'dark' | 'light' | 'system';
+
+export enum AuthState {
+    INVALID = 'invalid',
+    LOADING = 'loading',
+    VALID = 'valid',
+}