Fix ltx2 FFLF last image resizing

[atomantic]

Summary

• resize the ltx2 two-frame FFLF end frame before passing it to the helper
• keep legacy mlx_video/Windows last-image skip behavior intact
• remove the completed PLAN item and add a changelog note

Local review

• Checked runtime path: ltx2 true-FFLF now gets resized start and end images before buildArgs emits --image/--last-image.
• Checked regression coverage: test asserts both ffmpeg resize inputs and the final helper args.
• Findings: none.

Tests

• npm test --prefix server -- services/videoGen/local.test.js
• git diff --check