🎨 Palette: Enable native Enter-to-submit for cover URL input

[jspann21]

💡 What: Wrapped the custom URL input and "Add" button in CoverPickerModal.tsx in a <form> element, changed the button type to "submit", and handled submission via the form's onSubmit event instead of an onKeyDown listener on the input.

🎯 Why: Previously, the user had to explicitly press "Enter" while focused strictly on the input, or click the button. Using a <form> provides native web behavior, meaning the user can submit the input naturally across all devices and assistive technologies, making the component feel more native and intuitive.

📸 Before/After: No visible changes, but the underlying DOM now correctly uses a <form> tag.

♿ Accessibility: Native HTML forms automatically trigger submit on the Enter key regardless of which element inside the form has focus, and provide better hooks for screen readers to announce form submission.

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PR created automatically by Jules for task 11805158047522853008 started by @jspann21

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