Preserve the non-blank label invariant for generated field insertion

[brylie]

Problem

insertCollectionField(..., { generateUniqueLabel: true }) normalizes a generated label through nextAvailableFieldLabel but does not reject a blank normalized base. A future caller can therefore persist an empty field label, bypassing the Collection schema invariant introduced by #205.

The current FieldMenu caller passes "New field", so this is not user-reachable in the released UI. It remains a data-layer correctness gap.

Outcome

Generated field insertion preserves the same non-blank, trimmed, case-insensitive unique-label contract as every other field write.

Acceptance criteria

• A whitespace-only label with generateUniqueLabel: true throws ValidationError("Field label cannot be blank").
• The failed transaction leaves the Collection schema unchanged.
• Repeated non-blank generated insertion continues to produce New field, New field 2, and so on.

Evidence

• src/lib/data/collection-ops.ts: nextAvailableFieldLabel trims but does not reject an empty base.
• Only current call site is FieldMenu.svelte, which passes "New field".
• Discovered after the release of Field creation doesn't reject a duplicate field label #205 / PR Reject duplicate collection field labels #270.