feat(ST-35): simplify the memory-film flow

[afaryy]

Summary

• replace the mandatory plan-review sequence with a simple request, selected-media, generation, preview, and save flow
• keep browser media selection explicit, removable, limited to 15 files, and consent-gated
• extract the MP4 from the existing export ZIP for a playable preview, with native share or download fallback
• apply a compact mobile-first blue visual system and retain safe sound choices

Verification

• Web tests: 7 passed
• Web production build: passed
• Local visual smoke: desktop and 375px mobile layouts inspected; no browser console errors
• Final diff whitespace check: passed

Boundaries

• This PR does not claim whole-phone-library access, direct social posting, a completed ClickHouse runtime gate, or a hosted end-to-end journey.
• Original-song preview and regeneration remain separate product work.