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/NotificationToast.svelte

@@ -82,9 +82,9 @@
 	});
 </script>
 
-<!-- svelte-ignore a11y-click-events-have-key-events -->
-<!-- svelte-ignore a11y-no-static-element-interactions -->
 <div
+	role="status"
+	aria-live="polite"
 	class="group relative flex gap-2.5 text-left min-w-[var(--width)] w-full dark:bg-gray-850 dark:text-white bg-white text-black border border-gray-100 dark:border-gray-800 rounded-3xl px-4 py-3.5 cursor-pointer select-none"
 	on:dragstart|preventDefault
 	on:pointerdown={onPointerDown}