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/OnBoarding.svelte

@@ -87,15 +87,15 @@
 				<div class="flex justify-center mt-8">
 					<div class="flex flex-col justify-center items-center">
 						<button
-							aria-labelledby="get-started"
+							aria-label={$i18n.t('Get started')}
 							class="relative z-20 flex p-1 rounded-full bg-white/5 hover:bg-white/10 transition font-medium text-sm"
 							on:click={() => {
 								getStartedHandler();
 							}}
 						>
-							<ArrowRightCircle className="size-6" />
+							<ArrowRightCircle className="size-6" aria-hidden="true" />
 						</button>
-						<div id="get-started" class="mt-1.5 font-primary text-base font-medium">
+						<div class="mt-1.5 font-primary text-base font-medium" aria-hidden="true">
 							{$i18n.t(`Get started`)}
 						</div>
 					</div>