feat(studio): add aspect ratio selector

[miguel-heygen]

Summary

• Adds an aspect ratio dropdown to the studio header toolbar (between project name and Capture/Renders)
• Supports 6 presets: 16:9 (1920x1080), 9:16 (1080x1920), 1:1 (1080x1080), 4:5 (1080x1350), 4:3 (1440x1080), 21:9 (2560x1080)
• Selecting a preset patches data-width/data-height on the root composition element and refreshes the preview
• Auto-detects current composition dimensions from the iframe DOM on load and via stage-size postMessage events

How it works

1. AspectRatioSelector component renders a button showing the current ratio (e.g., "16:9") with a dropdown of presets
2. On selection, handleAspectRatioChange reads the composition source HTML, patches the root element's dimension attributes using the existing applyPatchByTarget utility, saves back via the API, and triggers a preview refresh
3. The player web component auto-detects the new dimensions from data-width/data-height and re-scales the preview

Test plan

• Open a 16:9 composition in the studio — selector shows "16:9"
• Switch to 9:16 — preview re-renders in portrait, HTML source shows updated data-width="1080" data-height="1920"
• Switch to 1:1 — preview shows square, selector shows "1:1"
• Open the source editor — confirm data-width/data-height attributes reflect the selected ratio
• Drill into a sub-composition, change ratio — correct file is patched
• Existing tests pass (271/271)