fix: Isolate negative prompts per generation job and persist them for LEGO workflows

[ChuxiJ]

Problem

Negative prompts can leak through global generation state. LEGO generation reads generationForm.negativePrompt globally, and Add Layer generation params omit negativePrompt, so stale exclusions can affect unrelated jobs and edit/regenerate cannot reliably restore the intended prompt context.

Root Cause

Negative prompting was added at form level, but not promoted to a per-job/per-clip generation contract across all workflows.

Solution

• Store negative prompts in each generation job's explicit params.
• Pass negative prompts through LEGO/Add Layer/MultiTrack regeneration paths.
• Ensure queue retry and project history preserve the negative prompt used for that job.
• Clear or isolate global form state between unrelated jobs.

Verification

• Add tests for two consecutive jobs with different negative prompts.
• Add retry/regenerate tests proving the original negative prompt is preserved.
• Verify Add Layer and LEGO requests include the expected negative prompt.

Files to Touch

• src/services/generationPipeline.ts
• src/store/generationStore.ts
• src/types/generation.ts
• src/components/generation/*