fix(a11y): add aria-labels to chat core components (#21709)

Add aria-labels to close, back, and action buttons across:
- Controls/Controls.svelte: close chat controls button
- ChatControls/Embeds.svelte: close embed button
- Overview/Node.svelte: favorite toggle button
- Overview/View.svelte: back and close overview buttons
- ShortcutsModal.svelte: close button
- ShareChatModal.svelte: close button
- ToolServersModal.svelte: close button
- Placeholder/FolderTitle.svelte: folder icon picker, folder options menu

WCAG: 4.1.2 (Name, Role, Value)

src/lib/components/chat/Overview/Node.svelte

@@ -56,6 +56,7 @@
 
 						<button
 							class={data?.message?.favorite ? '' : 'invisible group-hover:visible'}
+							aria-label={data?.message?.favorite ? $i18n.t('Remove from favorites') : $i18n.t('Add to favorites')}
 							on:click={() => {
 								data.message.favorite = !(data?.message?.favorite ?? false);
 							}}

src/lib/components/chat/Overview/View.svelte

@@ -176,6 +176,7 @@
 		<div class="flex items-center gap-2.5">
 			<button
 				class="self-center p-0.5"
+				aria-label={$i18n.t('Back')}
 				on:click={() => {
 					showOverview.set(false);
 				}}
@@ -186,6 +187,7 @@
 		</div>
 		<button
 			class="self-center p-0.5"
+			aria-label={$i18n.t('Close overview')}
 			on:click={() => {
 				onClose();
 				showOverview.set(false);