feat(perf): add loading.tsx skeletons for async marketing routes

Fixes #1243

Add sibling loading.tsx files for all 6 async route segments that were
missing instant loading UI, causing blank screens during navigation on
slow networks or cold caches.

Routes covered:
- /docs/[[...slug]]   — awaits getDocPage + MDX body
- /blog               — awaits source.getPages()
- /blog/[slug]        — awaits params + MDX body
- /changelog          — awaits source.getPages()
- /free               — awaits getModels() fetch
- /free/[model_slug]  — awaits Promise.all([getModel, getAllModels])

Each loading.tsx is a Server Component returning an animate-pulse
skeleton that matches its route's layout (header, content area,
grid/table/timeline as appropriate). Uses the Skeleton component and
Tailwind classes already present in the project.

Follows the pattern established in:
- app/dashboard/[search_space_id]/logs/loading.tsx
- app/dashboard/[search_space_id]/new-chat/loading.tsx

surfsense_web/app/(home)/free/[model_slug]/loading.tsx

@@ -0,0 +1,65 @@
+import { Skeleton } from "@/components/ui/skeleton";
+
+export default function FreeModelLoading() {
+	return (
+		<>
+			{/* Chat area skeleton - fills viewport */}
+			<div className="h-full flex flex-col">
+				{/* Chat header */}
+				<div className="flex items-center gap-3 border-b px-4 py-3">
+					<Skeleton className="h-8 w-8 rounded-full" />
+					<Skeleton className="h-5 w-40" />
+				</div>
+
+				{/* Chat messages area */}
+				<div className="flex-1 flex flex-col justify-end gap-4 px-4 py-6">
+					<div className="flex justify-end">
+						<Skeleton className="h-10 w-56 rounded-2xl" />
+					</div>
+					<div className="space-y-2 max-w-lg">
+						<Skeleton className="h-4 w-full" />
+						<Skeleton className="h-4 w-4/5" />
+						<Skeleton className="h-4 w-3/4" />
+					</div>
+				</div>
+
+				{/* Input bar */}
+				<div className="border-t px-4 py-3">
+					<Skeleton className="h-12 w-full rounded-xl" />
+				</div>
+			</div>
+
+			{/* SEO section skeleton */}
+			<div className="border-t bg-background">
+				<div className="container mx-auto px-4 py-10 max-w-3xl">
+					{/* Breadcrumb */}
+					<div className="flex items-center gap-2 mb-6">
+						<Skeleton className="h-4 w-10" />
+						<Skeleton className="h-4 w-3" />
+						<Skeleton className="h-4 w-24" />
+						<Skeleton className="h-4 w-3" />
+						<Skeleton className="h-4 w-32" />
+					</div>
+
+					<Skeleton className="h-7 w-3/4 mb-2" />
+					<Skeleton className="h-4 w-full mb-1" />
+					<Skeleton className="h-4 w-2/3 mb-8" />
+
+					<div className="my-8 h-px bg-border" />
+
+					{/* FAQ skeleton */}
+					<Skeleton className="h-6 w-64 mb-4" />
+					<div className="flex flex-col gap-3">
+						{Array.from({ length: 4 }).map((_, i) => (
+							<div key={i} className="rounded-lg border bg-card p-4 space-y-2">
+								<Skeleton className="h-4 w-3/4" />
+								<Skeleton className="h-3 w-full" />
+								<Skeleton className="h-3 w-5/6" />
+							</div>
+						))}
+					</div>
+				</div>
+			</div>
+		</>
+	);
+}