fix: update syncSecretsOptions condition and enhance documentation for mode configuration

- Changed the condition for syncSecretsOptions to check for both existence and false value.
- Added details about explicit fetch export and integration specifics in the introduction documentation.
- Clarified behavior of UI-only and code-only modes, including configuration application and schema syncing.

app/src/modes/shared.ts

@@ -129,7 +129,7 @@ export async function makeModeConfig<
          );
       }
 
-      if (syncSecretsOptions?.enabled) {
+      if (syncSecretsOptions && syncSecretsOptions.enabled !== false) {
          if (plugins.some((p) => p.name === "bknd-sync-secrets")) {
             throw new Error("You have to unregister the `syncSecrets` plugin");
          }