Phase 9C.2: share manual clip selection action

[FernandoAbishai]

Summary

• add a selection-aware manual clip CTA to Create Clips → Find
• centralize transcript-selection draft creation in one pure clip-domain helper
• reuse the same helper from TranscriptEditor and Create Clips

Scope

No backend, provider, project schema, or export authority changes.

Validation

• npm --prefix frontend run test:create-clips-workspace
• npm --prefix frontend run test:transcript-selection
• npm run smoke:golden-creator-path
• npm --prefix frontend run lint
• npm run build:frontend
• git diff --check

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