Add CSS appearance: base-select support for customizable select dropdowns

[Copilot]

Implements the new CSS customizable select spec (appearance: base-select) with styled dropdowns matching the app's design system when supported.

HTML Structure

• Added <button><selectedcontent></selectedcontent></button> inside selects (required by spec for popover activation)
• Added SVG icons with aria-hidden="true" to action menu options
• Removed <hr> separators from action menus

CSS (@supports (appearance: base-select))

• ::picker(select) — --shadow-6, surface colors, --radius-2, fade+scale transitions via @starting-style
• ::picker-icon — hidden
• option::checkmark — order: 2 for inline-end positioning
• option[disabled] — hidden
• option — gap: var(--size-3); padding: var(--size-2)
• overflow: hidden → overflow: clip on ancestors for anchor positioning

Svelte Reactivity

Base-select's popover doesn't trigger Svelte's binding detection. Added explicit oninput handlers:

<select bind:value={$store} oninput={(e) => $store = e.currentTarget.value}>
  <button><selectedcontent></selectedcontent></button>
  <option><svg aria-hidden="true">...</svg> Label</option>
</select>

Action Button Selects

.global-actions, .layer-actions, .stop-actions selects use opacity: 0; inset: 0; padding: 0 to fit parent and let background icon show through.

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