Harden build failures and dev CSS placeholders

Propagate webpack build failures via explicit exit codes
so production builds fail loudly.
Derive dev CSS placeholders from the computed output list
to avoid hardcoded filenames.

Author: PeterDaveHello

build.mjs

@@ -399,21 +399,34 @@ async function runWebpack(isWithoutKatex, isWithoutTiktoken, minimal, sourceBuil
   if (isProduction) {
     // Ensure compiler is properly closed after production runs
     compiler.run((err, stats) => {
+      const hasErrors = !!(
+        err ||
+        (stats && typeof stats.hasErrors === 'function' && stats.hasErrors())
+      )
+      let callbackFailed = false
       const finishClose = () =>
         compiler.close((closeErr) => {
           if (closeErr) {
             console.error('Error closing compiler:', closeErr)
             process.exitCode = 1
           }
+          if (hasErrors || callbackFailed) {
+            process.exitCode = 1
+          }
         })
       try {
         const ret = callback(err, stats)
         if (ret && typeof ret.then === 'function') {
-          ret.then(finishClose, finishClose)
+          ret.then(finishClose, () => {
+            callbackFailed = true
+            finishClose()
+          })
         } else {
           finishClose()
         }
-      } catch (_) {
+      } catch (err) {
+        console.error('[build] Callback error:', err)
+        callbackFailed = true
         finishClose()
       }
     })
@@ -510,18 +523,19 @@ async function copyFiles(entryPoints, targetDir) {
 }
 
 // In development, create placeholder CSS and sourcemap files to avoid 404 noise
-async function ensureDevCssPlaceholders(targetDir) {
-  if (isProduction) return
-  const cssFiles = [path.join(targetDir, 'popup.css'), path.join(targetDir, 'content-script.css')]
-  for (const cssPath of cssFiles) {
-    if (!(await fs.pathExists(cssPath))) {
-      await fs.outputFile(cssPath, '/* dev placeholder */\n')
-    }
-    const mapPath = `${cssPath}.map`
-    if (!(await fs.pathExists(mapPath))) {
-      await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
-    }
-  }
+async function ensureDevCssPlaceholders(cssFiles) {
+  if (isProduction || cssFiles.length === 0) return
+  await Promise.all(
+    cssFiles.map(async (cssPath) => {
+      if (!(await fs.pathExists(cssPath))) {
+        await fs.outputFile(cssPath, '/* dev placeholder */\n')
+      }
+      const mapPath = `${cssPath}.map`
+      if (!(await fs.pathExists(mapPath))) {
+        await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
+      }
+    }),
+  )
 }
 
 async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
@@ -560,7 +574,15 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
     [...commonFiles, { src: 'src/manifest.json', dst: 'manifest.json' }],
     chromiumOutputDir,
   )
-  await ensureDevCssPlaceholders(chromiumOutputDir)
+  await ensureDevCssPlaceholders(
+    Array.from(
+      new Set(
+        commonFiles
+          .filter((file) => file.dst.endsWith('.css'))
+          .map((file) => path.join(chromiumOutputDir, file.dst)),
+      ),
+    ),
+  )
   if (isProduction) await zipFolder(chromiumOutputDir)
 
   // firefox
@@ -569,7 +591,15 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
     [...commonFiles, { src: 'src/manifest.v2.json', dst: 'manifest.json' }],
     firefoxOutputDir,
   )
-  await ensureDevCssPlaceholders(firefoxOutputDir)
+  await ensureDevCssPlaceholders(
+    Array.from(
+      new Set(
+        commonFiles
+          .filter((file) => file.dst.endsWith('.css'))
+          .map((file) => path.join(firefoxOutputDir, file.dst)),
+      ),
+    ),
+  )
   if (isProduction) await zipFolder(firefoxOutputDir)
 }
 
@@ -605,10 +635,14 @@ async function build() {
     const tmpMin = `${outdir}/.tmp-min`
     try {
       if (parallelBuild) {
-        await Promise.all([
+        const results = await Promise.allSettled([
           createWebpackBuildPromise(true, true, true, tmpMin, '-without-katex-and-tiktoken'),
           createWebpackBuildPromise(false, false, false, tmpFull, ''),
         ])
+        const failed = results.find((result) => result.status === 'rejected')
+        if (failed) {
+          throw failed.reason
+        }
       } else {
         await createWebpackBuildPromise(true, true, true, tmpMin, '-without-katex-and-tiktoken')
         await createWebpackBuildPromise(false, false, false, tmpFull, '')