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/ShortcutsModal.svelte

@@ -47,7 +47,7 @@
 	<div class="text-gray-700 dark:text-gray-100 px-5 py-4">
 		<div class="flex justify-between dark:text-gray-300 pb-2">
 			<div class="text-lg font-medium self-center">{$i18n.t('Keyboard Shortcuts')}</div>
-			<button class="self-center" on:click={() => (show = false)}>
+			<button class="self-center" aria-label={$i18n.t('Close')} on:click={() => (show = false)}>
 				<XMark className={'size-5'} />
 			</button>
 		</div>