feat: add subdomain-based site context with SSR resolution

Resolve the active site from the request Host header on the server and
expose it through a SiteContext provider so the app can vary rendering
per subdomain. Serialize the resolved site into the document shell
(data-site attribute + window.__SITE__) so client hydration matches the
server render. Add host-based Vercel rewrites mapping each subdomain to
its site root, and include unit tests for the site resolution logic.

src/entry-server.tsx

@@ -1,19 +1,29 @@
 // @refresh reload
 import { createHandler, StartServer } from "@solidjs/start/server";
+import { getSiteFromEvent } from "~/server/site-context-server";
+import { setServerSite } from "~/context/SiteContext";
 
-export default createHandler(() => (
-  <StartServer
-    document={({ assets, children, scripts }) => (
-      <html lang="en">
-        <head>
-          <meta charset="utf-8" />
-          <meta
-            name="viewport"
-            content="width=device-width, initial-scale=1, maximum-scale=1"
-          />
-          <link rel="icon" href="/favicon.ico" />
-          <script>
-            {`
+export default createHandler((event) => {
+  // Resolve the active site from the request Host header once per SSR pass,
+  // then bind it so the SiteContext provider returns the right value during
+  // the initial server render. Also serialized into the document shell
+  // (`<html data-site>` + `window.__SITE__`) so client hydration matches.
+  const site = getSiteFromEvent(event);
+  setServerSite(site);
+
+  return (
+    <StartServer
+      document={({ assets, children, scripts }) => (
+        <html lang="en" data-site={site.id}>
+          <head>
+            <meta charset="utf-8" />
+            <meta
+              name="viewport"
+              content="width=device-width, initial-scale=1, maximum-scale=1"
+            />
+            <link rel="icon" href={site.faviconPath} />
+            <script>
+              {`
               (function() {
                 const STORAGE_KEY = 'theme-override';
                 const stored = localStorage.getItem(STORAGE_KEY);
@@ -22,14 +32,17 @@ export default createHandler(() => (
                 document.documentElement.classList.add(isDark ? 'dark' : 'light');
               })();
             `}
-          </script>
-          {assets}
-        </head>
-        <body>
-          <div id="app">{children}</div>
-          {scripts}
-        </body>
-      </html>
-    )}
-  />
-));
+            </script>
+            {/* Hydration data for SiteContext — must run before the app bundle. */}
+            <script>{`window.__SITE__=${JSON.stringify(site.id)};`}</script>
+            {assets}
+          </head>
+          <body>
+            <div id="app">{children}</div>
+            {scripts}
+          </body>
+        </html>
+      )}
+    />
+  );
+});