fix(a11y): improve accessibility of top-level auth and onboarding components (#21710)

Adds critical accessibility fixes across various app components:
- auth/+page: provide alt text for logo, turn on screenReader support for password input, add aria-required, hide decorative SVGs from AT
- AppSidebar: wrap navigation icons in a <nav> structure, provide ARIA labels for Home and Chat icons
- s/[id]/+page: convert structural divs into semantically accurate h1 heading and time element, wrap message display in main region
- OnBoarding: replace flawed aria-labelledby with direct aria-label on start button
- NotificationToast: provide role='status' and aria-live='polite' for proper screen reader broadcasting
- ChangelogModal: add required heading semantics for structure
- AddFilesPlaceholder: provide heading element role for standalone text content
- ImportModal: provide aria-label for close button

Addresses WCAG 4.1.3, 1.1.1, 3.3.2, and 1.3.1.

src/lib/components/ChangelogModal.svelte

@@ -38,11 +38,11 @@
 <Modal bind:show size="xl">
 	<div class="px-6 pt-5 dark:text-white text-black">
 		<div class="flex justify-between items-start">
-			<div class="text-xl font-medium">
+			<h2 class="text-xl font-medium m-0">
 				{$i18n.t("What's New in")}
 				{$WEBUI_NAME}
 				<Confetti x={[-1, -0.25]} y={[0, 0.5]} />
-			</div>
+			</h2>
 			<button class="self-center" on:click={closeModal} aria-label={$i18n.t('Close')}>
 				<XMark className={'size-5'}>
 					<p class="sr-only">{$i18n.t('Close')}</p>
@@ -64,9 +64,9 @@
 				{#if changelog}
 					{#each Object.keys(changelog) as version}
 						<div class=" mb-3 pr-2">
-							<div class="font-semibold text-xl mb-1 dark:text-white">
+							<h3 class="font-semibold text-xl mb-1 dark:text-white m-0">
 								v{version} - {changelog[version].date}
-							</div>
+							</h3>
 
 							<hr class="border-gray-50/50 dark:border-gray-850/50 my-2" />