🎨 Palette: Improve Run Simulations button UX

[google-labs-jules]

💡 What: Added a loading Spinner and an accessible wrapper to the disabled state of the "Run Simulations" button.
🎯 Why: Provides visual feedback during async operations (submitting the job) and gives clear, accessible guidance on why the button is disabled (needs exactly 4 decks selected).
📸 Before/After: Verified with Playwright testing locally.
♿ Accessibility: Ensures the disabled tooltip shows up on hover/focus by wrapping the disabled <button> in a block <span title="...">, avoiding issues where disabled buttons swallow pointer events.

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PR created automatically by Jules for task 7580896308862567612 started by @TytaniumDev

[google-labs-jules]

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