Embed testbed with tweaks (#9115)

* Nightly source-language update

* add: test embed page for bskyembeds

* run prettier

* wip

* feat: use local dev for bskyembed test.html

* feat: commit generated files which i'm not sure about

* chore: add basic readme to bskyembed

* Revert "feat: commit generated files which i'm not sure about"

This reverts commit b891efdaa1fd7ddd4417bfd5267e861a25a93b0e.

* chore: revert change to bskyembed/.eslintrc.cjs

* chore: remove generated file from bskyweb/static

* chore: revert messages.po changes

* add dev-snippet command, add dev router

* Update README.md

* Update bskyembed/test.html

* change IS_DEV check to allow localhost

---------

Co-authored-by: APiligrim <28320272+APiligrim@users.noreply.github.com>
Co-authored-by: Anastasiya Uraleva <anastasiya@Anastasiyas-MacBook-Pro.local>
Co-authored-by: Anastasiya Uraleva <anastasiya@Mac.localdomain>
Co-authored-by: Elijah Seed-Arita <elijaharita@gmail.com>

bskyembed/vite.config.ts

@@ -1,10 +1,40 @@
+import fs from 'node:fs'
 import {resolve} from 'node:path'
 
 import preact from '@preact/preset-vite'
 import legacy from '@vitejs/plugin-legacy'
-import type {UserConfig} from 'vite'
+import type {Plugin, UserConfig} from 'vite'
 import paths from 'vite-tsconfig-paths'
 
+/**
+ * World's hackiest router, for dev only. Serves `/post.html` to requests that start with `/embed/`
+ */
+function devOnlyRouter(): Plugin {
+  return {
+    name: 'embed-to-post-html',
+    configureServer(server) {
+      server.middlewares.use((req, res, next) => {
+        const url = req.url || ''
+        if (!url.startsWith('/embed/')) return next()
+
+        const html = fs.readFileSync(
+          resolve(process.cwd(), 'post.html'),
+          'utf8',
+        )
+
+        server
+          .transformIndexHtml(url, html)
+          .then(transformed => {
+            res.statusCode = 200
+            res.setHeader('Content-Type', 'text/html')
+            res.end(transformed)
+          })
+          .catch(next)
+      })
+    },
+  }
+}
+
 const config: UserConfig = {
   plugins: [
     preact(),
@@ -12,6 +42,7 @@ const config: UserConfig = {
     legacy({
       targets: ['defaults', 'not IE 11'],
     }),
+    devOnlyRouter(),
   ],
   build: {
     assetsDir: 'static',