Clearer account deletion and Obsidian sync fixes

Improved

• Delete account -- delete-account and Clear all context dialogs now list what each action actually removes. The leaving-reason picker uses the in-app select instead of the browser dropdown.
• Account deletion -- deleting your account now revokes MCP tokens and OAuth grants tied to your agents.

Fixed

• Delete account -- the leaving-reason list loads correctly so the confirm button is no longer stuck disabled.
• Obsidian sync -- uploads no longer fail when a note title exceeds 512 characters, or when one note in a batch is blank (empty notes are skipped instead of rejecting the whole batch).
• Memory -- reliability improvements for memory promotion after extraction.