fix: classify invalid image inputs

[Minorli]

Summary

• stop treating generic image_generation_user_error as content moderation
• classify invalid_mask_image_format as actionable upstream_invalid_image_input
• preserve explicit content policy codes and messages
• release version 0.1.59

Production evidence

A real mask edit returned invalid_mask_image_format / mask size does not match image size, but PicGen surfaced upstream_content_policy. Both uploaded PNGs were 1088x2240, indicating an upstream compatibility restriction rather than moderation.

Verification

• ./scripts/check.sh (389 passed)
• coverage: 87%
• explicit invalid-mask and content-policy counter-tests
• API response test for actionable input guidance
• node --check static/app.js
• bash -n scripts/*.sh
• docker compose config --quiet
• uv lock --check
• independent review: no findings