fix: resolve all 40 Svelte dev warnings for clean startup

- Add $state() to 4 reactive variables (guestMode, emailInput, passwordInput, searchInputElement)
- Replace 3 deprecated <svelte:component> with direct component references
- Fix 8 a11y issues: add ARIA roles, tabindex, keyboard handlers to click-handler divs
- Remove 22 unused CSS selectors across 8 shared-ui components

Zero warnings on dev startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/shared-ui/src/dnd/ActionZone.svelte

@@ -60,7 +60,8 @@
 		role="button"
 		tabindex="-1"
 	>
-		<svelte:component this={iconComponent} size={20} weight="bold" />
+		{@const Icon = iconComponent}
+		<Icon size={20} weight="bold" />
 		{#if label}
 			<span class="action-label">{label}</span>
 		{/if}