feat: improve canvas media references and motion control

[erickkkyt]

Summary

• detect image and video uploads automatically and render both media types correctly on Canvas
• keep cards movable and persist completed drag positions
• add semantic @ImageN and @VideoN references with thumbnails, ordering, insertion, and prompt alias updates
• preserve multiple reference videos when building BeatAPI generation requests
• add Kling 2.6 and Kling 3.0 Motion Control models, parameters, upload validation, pricing, icons, docs, and tests

Validation

• pnpm typecheck
• pnpm test (167 passed)
• pnpm i18n:check
• pnpm build

Release boundary

Local public/demo-assets/ files are intentionally excluded from GitHub.