Improve image and video generation loading states

[Chel-Tbl]

Summary

• add a shared animated loading state for generation tools
• replace the plain prompt-only image/video generation call UIs with visual previews, sweep animation, prompt text, and progress bars
• add small CSS keyframes for the generation loading animations

Verification

• tsc --noEmit --pretty false
• targeted prettier --check for changed generation loading files
• git diff --check
• SKIP_ENV_VALIDATION=1 next build

Closes #207