🎨 Palette: Async Form Button Loading States

[singhaditya21]

• Added explicitly accessible and visual loading states to handleLogin, handleRegister, and handleGoogleSignIn buttons in web-demo/js/app.js.
• Converted setTimeout to awaitable Promise inside handleGoogleSignIn to allow proper tracking with a try/finally block.
• Updated palette.md journal with learning about try/finally blocks and button state restorations.
• All loading states verify by setting disabled, adding aria-busy=true and replacing content to indicate progress.

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PR created automatically by Jules for task 14848538476726881230 started by @singhaditya21

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