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)/blog/loading.tsx

@@ -0,0 +1,50 @@
+import { Skeleton } from "@/components/ui/skeleton";
+
+export default function BlogIndexLoading() {
+	return (
+		<div className="relative overflow-hidden bg-neutral-50 px-4 pt-20 md:px-8 dark:bg-neutral-950">
+			<div className="mx-auto max-w-6xl pt-12 pb-24 md:pt-20">
+				{/* Header */}
+				<div className="mb-10 md:mb-14">
+					<Skeleton className="h-10 w-24 rounded-md" />
+				</div>
+
+				{/* Featured post skeleton */}
+				<div className="mb-14 overflow-hidden rounded-3xl border border-neutral-200/80 dark:border-neutral-800">
+					<Skeleton className="aspect-[2.4/1] min-h-[220px] w-full rounded-none" />
+					<div className="p-6 md:p-8 space-y-3">
+						<Skeleton className="h-5 w-24 rounded-full" />
+						<Skeleton className="h-8 w-3/4" />
+						<Skeleton className="h-4 w-full max-w-lg" />
+						<div className="flex items-center gap-3 pt-2">
+							<Skeleton className="h-8 w-8 rounded-full" />
+							<Skeleton className="h-4 w-28" />
+							<Skeleton className="h-4 w-20" />
+						</div>
+					</div>
+				</div>
+
+				{/* Search bar skeleton */}
+				<div className="mb-10">
+					<Skeleton className="h-11 w-full max-w-md rounded-full" />
+				</div>
+
+				{/* Grid of article cards */}
+				<div className="grid gap-8 md:grid-cols-2 lg:grid-cols-3">
+					{Array.from({ length: 6 }).map((_, i) => (
+						<div key={i} className="space-y-3">
+							<Skeleton className="aspect-video w-full rounded-2xl" />
+							<Skeleton className="h-5 w-3/4" />
+							<Skeleton className="h-4 w-full" />
+							<Skeleton className="h-4 w-5/6" />
+							<div className="flex items-center gap-2 pt-1">
+								<Skeleton className="h-6 w-6 rounded-full" />
+								<Skeleton className="h-4 w-24" />
+							</div>
+						</div>
+					))}
+				</div>
+			</div>
+		</div>
+	);
+}