Fix blog build + clear 14 dependabot advisories

Build was failing because the new MDX post tried to `import` the
SuchMoonLaunchStoreButtons component — next-mdx-remote/rsc doesn't
support imports inside .mdx; components must be passed via the
`components` prop. Wire the helper into [slug]/page.tsx and drop the
import line from the .mdx.

Dependency bumps to clear the 14 outstanding Dependabot advisories
(9 high, 4 moderate, 1 low):
- apps/web/next: ^15.5.15 → ^15.5.18 (covers 12 next.js advisories)
- pnpm overrides: pin postcss ^8.5.14 (clears 1117015 XSS via </style>)
- pnpm overrides: pin fast-uri ^3.1.2 (clears 1117870 + 1117884 path
  traversal / host confusion)

pnpm audit: 14 vulnerabilities → 0.

apps/web/package.json

@@ -18,7 +18,7 @@
     "framer-motion": "^11.0.0",
     "gray-matter": "^4.0.3",
     "lucide-react": "^0.350.0",
-    "next": "^15.5.15",
+    "next": "^15.5.18",
     "next-mdx-remote": "^6.0.0",
     "next-themes": "^0.4.6",
     "react": "^19.2.4",