Design the large-label-set confirmation dialog

## Question

Design the confirmation dialog: copy, exactly when it appears, "don't ask again"/persistence behavior, and Cancel vs. Synchronize actions.

Draft copy from Slack:
> The database contains [count] combined unique node and edge labels. This can lead to a degraded experience.
> [Cancel] [Synchronize Schema]

Blocked by the count-source/interjection decision and the threshold — the dialog can only be designed once we know what count we can show and what number is 'excessive'.

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> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.

Start by resolving the count-source/interjection decision and the threshold, since the dialog design is blocked on both. Then define the copy, appearance timing, persistence for “don't ask again,” and Cancel versus Synchronize actions; done means these behaviors and the displayed count are agreed.

Written by the indexing model from the issue text.