fix(studio): keep canvas chrome off scaled iframe

[ryansiyglobalcom]

What

Moves the Studio preview outline and drop shadow from the scaled composition iframe to the outer preview zoom stage. Adds regression coverage for both ownership boundaries.

Why

At fractional fit scales, the iframe box shadow is rasterized on top of composition pixels and appears as a scene-dependent one-pixel edge seam or flicker. Studio chrome should frame the canvas without modifying the composition surface.

How

Removes the shadow-root CSS injection from Player and applies the same visual treatment to preview-zoom-stage, outside the composition iframe. Rendered output and composition code are unchanged.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable: internal editor styling change)

Verification:

• Full monorepo build
• Full Studio test suite: 4,838 passed, 18 todo; 438 files passed, 1 skipped
• Studio typecheck
• Full repository lint and changed-file format checks
• Fallow audit against origin/main
• Browser check at a fractional 1098.65625px preview width: iframe has no box shadow; stage owns the outline/drop shadow; composition edge remains clean

[miguel-heygen]

@ryansiyglobalcom can you attach in the pr description before vs after screenshots?