fix: match the API limit of five Lab comment images

[ClarenceStark]

The Lab comment input allows six pictures from either the picker or clipboard, while both CreateLabModelComment and UpdateLabModelComment in compass-web-service reject more than five. Users can therefore prepare a comment that the API will always reject.

Use one five-image limit for both input paths and update the warning. Existing images loaded into the edit form count toward the same limit.

Validation: 6 component tests pass in Jest/Node 22.12.0 (baseline 4 failed, 2 passed). Each input route rejects six new images, accepts five, and rejects adding two to an existing four. File conversion and image preview are isolated; live upload/API submission was not exercised.

Backend contract: app/graphql/mutations/create_lab_model_comment.rb and update_lab_model_comment.rb at oss-compass/compass-web-service main.

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