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/ChatControls/Embeds.svelte

@@ -57,6 +57,7 @@
 
 			<button
 				class="self-center pointer-events-auto p-1 rounded-full bg-white dark:bg-gray-850"
+				aria-label="Close embed"
 				on:click={() => {
 					showControls.set(false);
 					showEmbeds.set(false);