Harden Sass fallback when sass-embedded unavailable

If both 'sass-embedded' and 'sass' fail to load, fail fast with a clear
error message instead of surfacing an opaque dynamic import error. Keeps
build failures actionable in misconfigured environments.

Author: PeterDaveHello

build.mjs

@@ -98,9 +98,13 @@ async function runWebpack(isWithoutKatex, isWithoutTiktoken, minimal, sourceBuil
   try {
     const mod = await import('sass-embedded')
     sassImpl = mod.default || mod
-  } catch (e) {
-    const mod = await import('sass')
-    sassImpl = mod.default || mod
+  } catch (e1) {
+    try {
+      const mod = await import('sass')
+      sassImpl = mod.default || mod
+    } catch (e2) {
+      throw new Error(`No Sass implementation available. Install 'sass-embedded' or 'sass'.`)
+    }
   }
 
   const dirKey = path.basename(sourceBuildDir || outdir)