fix(apps): persist auto-select knowledge base checkbox

[squarezw]

Summary

• Persist the “自动选择知识库” checkbox in settings.auto_select_datasets instead of inferring it from empty dataset_ids.
• Restore the checkbox from that flag on edit, so unchecking it without binding datasets still survives a reopen.
• Pair with ragent-service fix/honor-app-auto-select-datasets so chat actually stops auto-selecting when the flag is off.

Test plan

• Edit an app, uncheck “自动选择知识库”, save without picking datasets, reopen — checkbox stays off and the dataset list is shown
• Check it again, save, reopen — checkbox is on
• Bind specific datasets with the checkbox off, save, reopen — selections and unchecked state remain
• Existing apps that never saved the flag still treat empty dataset_ids as auto-select

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