[Embeds] Embed subdomain landing page (#3501)

* add build output to web build

* simplify post-build step by copying everything at once

* make script that converts placeholder -> iframe

* dynamically resize iframe based on inner content

Requires the iframe content to `postMessage` its height back up to the parent

* add lang to embed

* svg explicit height -> viewBox

* add build output to web build

* simplify post-build step by copying everything at once

* attempt to fix go embed issue

* rm changes to bskyweb

* remove another bskyweb change

* embed landing page

* Drop xl breakpoint, too far down

* Remove pointer enter behavior

* Avoid button width jump

* Escape HTML

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Co-authored-by: Dan Abramov <dan.abramov@gmail.com>

bskyembed/package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc && vite build",
+    "build-snippet": "tsc --project tsconfig.snippet.json",
     "lint": "eslint --cache --ext .js,.jsx,.ts,.tsx src"
   },
   "dependencies": {