feat: make clips a first-class workspace

[FernandoAbishai]

Summary

• make Create Clips the primary short-workflow surface while keeping generic AI tools secondary
• reset media-specific clip state safely across new media, including stale async-write protection
• restore Clips context and derived stage when reopening projects with saved clip workspace data
• disambiguate full-recording Export as Export Video in short workflow

Verification

• all frontend smoke scripts pass
• npm run lint
• npm run build:frontend
• git diff --check

Scope

Phase 7C.2 Slice 1 only. No backend, schema, discovery, review, export backend, release, or alpha.5 changes. Do not merge.

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