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/routes/auth/+page.svelte

@@ -243,7 +243,7 @@
 										crossorigin="anonymous"
 										src="{WEBUI_BASE_URL}/static/favicon.png"
 										class="size-24 rounded-full"
-										alt=""
+										alt="{$WEBUI_NAME} logo"
 									/>
 								</div>
 							{/if}
@@ -342,8 +342,9 @@
 												placeholder={$i18n.t('Enter Your Password')}
 												autocomplete={mode === 'signup' ? 'new-password' : 'current-password'}
 												name="password"
-												screenReader={false}
+												screenReader={true}
 												required
+												aria-required="true"
 											/>
 										</div>
 
@@ -439,6 +440,7 @@
 												xmlns="http://www.w3.org/2000/svg"
 												viewBox="0 0 48 48"
 												class="size-6 mr-3"
+												aria-hidden="true"
 											>
 												<path
 													fill="#EA4335"
@@ -468,6 +470,7 @@
 												xmlns="http://www.w3.org/2000/svg"
 												viewBox="0 0 21 21"
 												class="size-6 mr-3"
+												aria-hidden="true"
 											>
 												<rect x="1" y="1" width="9" height="9" fill="#f25022" /><rect
 													x="1"
@@ -498,6 +501,7 @@
 												xmlns="http://www.w3.org/2000/svg"
 												viewBox="0 0 24 24"
 												class="size-6 mr-3"
+												aria-hidden="true"
 											>
 												<path
 													fill="currentColor"
@@ -521,6 +525,7 @@
 												stroke-width="1.5"
 												stroke="currentColor"
 												class="size-6 mr-3"
+												aria-hidden="true"
 											>
 												<path
 													stroke-linecap="round"

src/routes/s/[id]/+page.svelte

@@ -161,19 +161,22 @@
 						: 'max-w-5xl'} mx-auto"
 				>
 					<div class="px-3">
-						<div class=" text-2xl font-medium line-clamp-1">
+						<h1 class=" text-2xl font-medium line-clamp-1 m-0">
 							{title}
-						</div>
+						</h1>
 
 						<div class="flex text-sm justify-between items-center mt-1">
-							<div class="text-gray-400">
+							<time
+								class="text-gray-400"
+								datetime={new Date(chat?.chat?.timestamp || Date.now()).toISOString()}
+							>
 								{dayjs(chat.chat.timestamp).format('LLL')}
-							</div>
+							</time>
 						</div>
 					</div>
 				</div>
 
-				<div class=" h-full w-full flex flex-col py-2">
+				<div class=" h-full w-full flex flex-col py-2" role="main">
 					<div class="w-full">
 						<Messages
 							className="h-full flex pt-4 pb-8 "