Handle disallowed negative characteristics in gear submissions

[Perdolique]

Outcome

Users can correct disallowed negative characteristic values before submitting missing gear instead of receiving a generic retry message.

User behavior

• A negative value for a characteristic that does not allow negatives is identified at that field before submission.
• A negative value remains valid for characteristics that explicitly allow it, such as sleeping bag temperature ratings.
• Other entered submission data remains available while the value is corrected.

Acceptance criteria

• Disallowed negative values are not submitted.
• The affected characteristic shows a clear, specific validation message.
• Allowed negative values can still be submitted.
• Regression coverage protects both cases.

Dependencies / Out of scope

Follow-up technical debt from #718.

The server remains authoritative for validation. Changing which characteristics allow negative values is out of scope.