feat(loader): Use region-aware CDN host for Loader Script (#17476)

## DESCRIBE YOUR PR

The Loader Script snippet in the JavaScript install docs was hardcoded
to `https://js.sentry-cdn.com/...`, which is wrong for organizations
hosted in the EU (they need `js-de.sentry-cdn.com`). This PR makes the
host dynamic for signed-in users and adds a short alert for
unauthenticated readers.

- Add a new `JS_SDK_LOADER_HOST` code keyword on `ProjectCodeKeywords`,
derived from the project's DSN host (`.ingest.de.` →
`js-de.sentry-cdn.com`, else `js.sentry-cdn.com`).
- Swap the 5 loader `<script>` URLs in
`docs/platforms/javascript/common/install/loader.mdx` to use
`___JS_SDK_LOADER_HOST___`.
- Register `SignedInCheck` as an MDX component and wrap a small
`<Alert>` above the loader snippets telling unauthenticated readers to
swap to `js-de.sentry-cdn.com` if their org is in the EU.

Refs #17471

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## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: sergical

docs/platforms/javascript/common/install/loader.mdx

@@ -47,7 +47,7 @@ To use the loader, go in the Sentry UI to **Settings > Projects > (select projec
 
 ```html
 <script
-  src="https://js.sentry-cdn.com/___PUBLIC_KEY___.min.js"
+  src="https://___JS_SDK_LOADER_HOST___/___PUBLIC_KEY___.min.js"
   crossorigin="anonymous"
 ></script>
 ```
@@ -91,7 +91,7 @@ Alternatively, you can set the loader to request the full SDK earlier: still as
 
 ```html
 <script
-  src="https://js.sentry-cdn.com/___PUBLIC_KEY___.min.js"
+  src="https://___JS_SDK_LOADER_HOST___/___PUBLIC_KEY___.min.js"
   crossorigin="anonymous"
   data-lazy="no"
 ></script>
@@ -140,7 +140,7 @@ The loader script always includes a call to `Sentry.init` with a default configu
 </script>
 
 <script
-  src="https://js.sentry-cdn.com/___PUBLIC_KEY___.min.js"
+  src="https://___JS_SDK_LOADER_HOST___/___PUBLIC_KEY___.min.js"
   crossorigin="anonymous"
 ></script>
 ```
@@ -183,7 +183,7 @@ If you want to call any other method when using the Loader, you have to guard it
   };
 </script>
 
-<script src="https://js.sentry-cdn.com/___PUBLIC_KEY___.min.js" crossorigin="anonymous"></script>
+<script src="https://___JS_SDK_LOADER_HOST___/___PUBLIC_KEY___.min.js" crossorigin="anonymous"></script>
 
 <script>
   // Guard against window.Sentry not being available, e.g. due to Ad-blockers
@@ -220,7 +220,7 @@ If this is a critical issue for you, you have two options to ensure that all you
     };
   </script>
 
-  <script src="https://js.sentry-cdn.com/___PUBLIC_KEY___.min.js" crossorigin="anonymous"></script>
+  <script src="https://___JS_SDK_LOADER_HOST___/___PUBLIC_KEY___.min.js" crossorigin="anonymous"></script>
 
   <script>
     Sentry.onLoad(function () {

src/components/codeContext.tsx

@@ -9,6 +9,7 @@ import {OnboardingOptionType} from './onboarding';
 type ProjectCodeKeywords = {
   API_URL: string;
   DSN: string;
+  JS_SDK_LOADER_HOST: string;
   MINIDUMP_URL: string;
   ORG_ID: number;
   ORG_INGEST_DOMAIN: string;
@@ -86,6 +87,7 @@ export const DEFAULTS: CodeKeywords = {
       ORG_ID: 0,
       ORG_SLUG: 'example-org',
       ORG_INGEST_DOMAIN: 'o0.ingest.sentry.io',
+      JS_SDK_LOADER_HOST: 'js.sentry-cdn.com',
       MINIDUMP_URL:
         'https://o0.ingest.sentry.io/api/0/minidump/?sentry_key=examplePublicKey',
       UNREAL_URL: 'https://o0.ingest.sentry.io/api/0/unreal/examplePublicKey/',
@@ -171,6 +173,10 @@ const formatApiUrl = ({scheme, host}: Dsn) => {
   return `${scheme}${apiHost}/api`;
 };
 
+const getJsSdkLoaderHost = ({host}: Dsn) => {
+  return host.includes('.ingest.de.') ? 'js-de.sentry-cdn.com' : 'js.sentry-cdn.com';
+};
+
 function getHost(): string {
   if (process.env.NODE_ENV === 'development') {
     return 'http://dev.getsentry.net:8000';
@@ -254,6 +260,7 @@ export async function fetchCodeKeywords(): Promise<CodeKeywords> {
         ORG_SLUG: project.organizationSlug,
         ORG_INGEST_DOMAIN:
           parsedDsn.host ?? `o${project.organizationId}.ingest.sentry.io`,
+        JS_SDK_LOADER_HOST: getJsSdkLoaderHost(parsedDsn),
         MINIDUMP_URL: formatMinidumpURL(parsedDsn),
         UNREAL_URL: formatUnrealEngineURL(parsedDsn),
         OTLP_URL: formatOtlpUrl(parsedDsn),