Video clips on the timeline previously had a transparent background,
which made them blend into the timeline surface and only become
visible when selected. Image clips share the same track type as
video and therefore had the same problem.
This adds a per-element-type tint layer that applies a subtle
background colour to clips on the timeline:
- video clips: sky tint (\`bg-sky-500/20\`)
- image clips: amber tint (\`bg-amber-500/20\`)
- audio / text / sticker / graphic / effect: unchanged, still
driven by \`TIMELINE_TRACK_THEME\`
The new \`TIMELINE_ELEMENT_THEME\` map is a Partial<Record<ElementType, …>>
so the element-level theme can override per type while keeping the
existing track-level theme as the fallback. A small unit test covers
the override and fallback behaviour.
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