fix(landing): use official platform brand icons via simple-icons

The hand-rolled Linux SVG path wasn't actually Tux — it was a symmetric
placeholder shape. Apple/Windows were close but not canonical either.

- Apple + Linux: pulled from @icons-pack/react-simple-icons (SiApple, SiLinux).
- Windows: simple-icons drops the Microsoft mark over trademark policy, so
  the Windows 11 flag is inlined from Microsoft's public brand guidance.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

landing/package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@fontsource/space-grotesk": "^5.2.10",
+    "@icons-pack/react-simple-icons": "^13.13.0",
     "@radix-ui/react-separator": "^1.1.8",
     "@radix-ui/react-slot": "^1.2.4",
     "autoprefixer": "^10.4.17",