fix(a11y): add aria-labels to chat message components (#21708)

Add aria-labels, aria-expanded, and semantic improvements to:
- RateComment: close button, rating scale, feedback textarea
- Citations: toggle button with count, source item buttons
- Source/SourceToken: contextual aria-labels for citation buttons
- StatusHistory: toggle button with expanded state
- WebSearchResults: descriptive favicon alt text
- FollowUps: convert div to button element
- RegenerateMenu: submit suggestion button
- FloatingButtons: action buttons, input field, submit button
- CitationModal: close button

WCAG: 4.1.2 (Name, Role, Value), 2.1.1 (Keyboard), 1.1.1 (Non-text Content)

src/lib/components/chat/Messages/Citations/CitationModal.svelte

@@ -130,6 +130,7 @@
 			</div>
 			<button
 				class="self-center"
+				aria-label={$i18n.t('Close citation modal')}
 				on:click={() => {
 					show = false;
 				}}