Fix sync TUI freeze by completing auth before starting dashboard

src/services/microsoft_graph/auth.py

@@ -25,6 +25,49 @@ def ensure_directory_exists(path):
         os.makedirs(path)
 
 
+def has_valid_cached_token(scopes=None):
+    """
+    Check if we have a valid cached token (without triggering auth flow).
+
+    Args:
+        scopes: List of scopes to check. If None, uses default scopes.
+
+    Returns:
+        bool: True if a valid cached token exists, False otherwise.
+    """
+    if scopes is None:
+        scopes = ["https://graph.microsoft.com/Mail.Read"]
+
+    client_id = os.getenv("AZURE_CLIENT_ID")
+    tenant_id = os.getenv("AZURE_TENANT_ID")
+
+    if not client_id or not tenant_id:
+        return False
+
+    cache = msal.SerializableTokenCache()
+    cache_file = "token_cache.bin"
+
+    if not os.path.exists(cache_file):
+        return False
+
+    try:
+        cache.deserialize(open(cache_file, "r").read())
+        authority = f"https://login.microsoftonline.com/{tenant_id}"
+        app = msal.PublicClientApplication(
+            client_id, authority=authority, token_cache=cache
+        )
+        accounts = app.get_accounts()
+
+        if not accounts:
+            return False
+
+        # Try silent auth - this will return None if token is expired
+        token_response = app.acquire_token_silent(scopes, account=accounts[0])
+        return token_response is not None and "access_token" in token_response
+    except Exception:
+        return False
+
+
 def get_access_token(scopes):
     """
     Authenticate with Microsoft Graph API and obtain an access token.