🧹 Refactor Studio page into smaller components

[pisandelli]

🎯 What: Extracted the monolithic runtime/pages/studio.vue into smaller, more focused components (StudioHeader, StudioSidebar, and StudioField).
💡 Why: This drastically improves the maintainability and readability of studio.vue by moving the complex view logic and specific implementations down into dedicated child components.
✅ Verification: Verified the refactor did not break the Vue component hierarchy. I passed data down via props rather than using events in the render function (which is an anti-pattern), avoiding warnings and keeping logic robust. I also verified the test suite executes successfully with no regressions.
✨ Result: The studio.vue page is now highly readable, primarily managing layout and state from the overarching theme editor composable.

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PR created automatically by Jules for task 11470221031158855589 started by @pisandelli

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