Summary

Add a "Placeholder" text input to the select options settings panel that manages
a special disabled/hidden first <option> element. The default select template
now ships with a "Select an option" placeholder out of the box.

Changes

• Add isPlaceholder flag to LayerSettings type for marking placeholder options
• Add placeholder input to SelectOptionsSettings with add/update/remove logic
• Update default select template to include a placeholder option
• Strip selected attribute from <option> elements in the React canvas renderer to prevent console warnings
• Set defaultValue="" on <select> elements with placeholder children in the canvas
• Add selected attribute for placeholder options in SSR HTML output

Test plan

• Add a new Select element from the elements panel — verify it shows "Select an option" placeholder by default
• Edit the placeholder text in the Options settings — verify it updates in the canvas
• Clear the placeholder text — verify the placeholder option is removed
• Publish and verify the placeholder shows correctly on the live page
• Select a different option, then check the placeholder cannot be re-selected (disabled + hidden)
• Verify no React console warnings about selected on <option>
• Verify collection-sourced selects still work correctly

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