feat(start): support serving assets via relative base urls

packages/react-router/src/Asset.tsx

@@ -136,7 +136,13 @@ function Script({
       const normSrc = (() => {
         try {
           const base = document.baseURI || window.location.href
-          return new URL(attrs.src, base).href
+          // If the src is explicitly relative and base is present, construct it carefully
+          // Without returning absolute URLs that break iframe/embedded deployments
+          const url = new URL(attrs.src, base)
+          if (attrs.src.startsWith('./') && !base.startsWith('http')) {
+            return attrs.src
+          }
+          return url.href
         } catch {
           return attrs.src
         }

packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts

@@ -263,7 +263,14 @@ export function createManifestAssetResolvers(
       return cachedPath
     }
 
-    const assetPath = joinURL(basePath, fileName)
+    let assetPath = joinURL(basePath, fileName)
+    if (basePath === './') {
+      // Avoid inserting a leading slash if base path is purely relative
+      assetPath = './' + fileName
+    } else if (basePath.startsWith('./') && assetPath.startsWith('/')) {
+      assetPath = '.' + assetPath
+    }
+
     assetPathByFileName.set(fileName, assetPath)
     return assetPath
   }