feat(web): add loading skeletons, error boundaries, and accessibility improvements

- Add segment-level loading.tsx for dashboard, search, admin, and auth routes
- Add segment-level error.tsx with Vietnamese error messages for all route groups
- Add skip-to-content navigation link in root layout
- Add id="main-content" to all layout main elements
- Add aria-label to nav elements and mobile menu buttons
- Improve dashboard nav responsiveness (icon-only on mobile)
- Hide user name on small screens in dashboard layout

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/(auth)/loading.tsx

@@ -0,0 +1,40 @@
+export default function AuthLoading() {
+  return (
+    <div className="rounded-lg border bg-card p-8 shadow-sm">
+      <div className="space-y-6">
+        {/* Logo / title skeleton */}
+        <div className="text-center">
+          <div className="mx-auto h-8 w-24 animate-pulse rounded bg-muted" />
+          <div className="mx-auto mt-3 h-5 w-40 animate-pulse rounded bg-muted" />
+          <div className="mx-auto mt-2 h-4 w-56 animate-pulse rounded bg-muted" />
+        </div>
+
+        {/* Form fields skeleton */}
+        <div className="space-y-4">
+          <div>
+            <div className="h-4 w-16 animate-pulse rounded bg-muted" />
+            <div className="mt-2 h-10 w-full animate-pulse rounded-md bg-muted" />
+          </div>
+          <div>
+            <div className="h-4 w-20 animate-pulse rounded bg-muted" />
+            <div className="mt-2 h-10 w-full animate-pulse rounded-md bg-muted" />
+          </div>
+        </div>
+
+        {/* Submit button skeleton */}
+        <div className="h-10 w-full animate-pulse rounded-md bg-muted" />
+
+        {/* OAuth buttons skeleton */}
+        <div className="space-y-3">
+          <div className="flex items-center gap-3">
+            <div className="h-px flex-1 bg-muted" />
+            <div className="h-3 w-12 animate-pulse rounded bg-muted" />
+            <div className="h-px flex-1 bg-muted" />
+          </div>
+          <div className="h-10 w-full animate-pulse rounded-md bg-muted" />
+          <div className="h-10 w-full animate-pulse rounded-md bg-muted" />
+        </div>
+      </div>
+    </div>
+  );
+}