[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

---------

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

bskyembed/vite.config.ts

@@ -1,3 +1,5 @@
+import {resolve} from 'node:path'
+
 import preact from '@preact/preset-vite'
 import legacy from '@vitejs/plugin-legacy'
 import type {UserConfig} from 'vite'
@@ -12,7 +14,13 @@ const config: UserConfig = {
     }),
   ],
   build: {
-    assetsDir: 'static/embed/assets',
+    assetsDir: 'static',
+    rollupOptions: {
+      input: {
+        index: resolve(__dirname, 'index.html'),
+        post: resolve(__dirname, 'post.html'),
+      },
+    },
   },
 }