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/[slug]/loading.tsx

@@ -0,0 +1,66 @@
+import { Skeleton } from "@/components/ui/skeleton";
+
+export default function BlogPostLoading() {
+	return (
+		<div className="min-h-screen relative pt-20">
+			<div className="max-w-3xl mx-auto px-6 lg:px-10 pt-10 pb-20">
+				{/* Breadcrumb */}
+				<div className="flex items-center gap-2 mb-8">
+					<Skeleton className="h-4 w-10" />
+					<Skeleton className="h-4 w-3" />
+					<Skeleton className="h-4 w-10" />
+					<Skeleton className="h-4 w-3" />
+					<Skeleton className="h-4 w-40" />
+				</div>
+
+				{/* Tags */}
+				<div className="flex flex-wrap gap-2 mb-4">
+					<Skeleton className="h-6 w-16 rounded-full" />
+					<Skeleton className="h-6 w-20 rounded-full" />
+				</div>
+
+				{/* Title */}
+				<div className="space-y-3 mb-6">
+					<Skeleton className="h-10 w-full" />
+					<Skeleton className="h-10 w-4/5" />
+				</div>
+
+				{/* Description */}
+				<Skeleton className="h-5 w-full mb-2" />
+				<Skeleton className="h-5 w-3/4 mb-8" />
+
+				{/* Author + date */}
+				<div className="flex items-center gap-3 mb-10">
+					<Skeleton className="h-10 w-10 rounded-full" />
+					<div className="space-y-1.5">
+						<Skeleton className="h-4 w-32" />
+						<Skeleton className="h-3 w-24" />
+					</div>
+				</div>
+
+				{/* Cover image */}
+				<Skeleton className="w-full aspect-video rounded-xl mb-10" />
+
+				{/* Article body paragraphs */}
+				{Array.from({ length: 5 }).map((_, i) => (
+					<div key={i} className="space-y-2 mb-6">
+						<Skeleton className="h-4 w-full" />
+						<Skeleton className="h-4 w-full" />
+						<Skeleton className="h-4 w-4/5" />
+					</div>
+				))}
+
+				{/* Sub-heading */}
+				<Skeleton className="h-7 w-56 mt-8 mb-4" />
+
+				{Array.from({ length: 3 }).map((_, i) => (
+					<div key={i} className="space-y-2 mb-6">
+						<Skeleton className="h-4 w-full" />
+						<Skeleton className="h-4 w-11/12" />
+						<Skeleton className="h-4 w-3/4" />
+					</div>
+				))}
+			</div>
+		</div>
+	);
+}