Refactor: extract helper for dev CSS placeholders

Reduce duplication in finishOutput by introducing ensureDevCssPlaceholders()
used by both Chromium and Firefox outputs. This keeps behavior identical while
making the build script easier to maintain and reason about. No functional
changes; verified via prettier, ESLint, and a production build.

Author: PeterDaveHello

build.mjs

@@ -484,6 +484,21 @@ 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 = `${cssPath}.map`
+    if (!(await fs.pathExists(mapPath))) {
+      await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
+    }
+  }
+}
+
 async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
   const commonFiles = [
     { src: 'src/logo.png', dst: 'logo.png' },
@@ -520,22 +535,7 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
     [...commonFiles, { src: 'src/manifest.json', dst: 'manifest.json' }],
     chromiumOutputDir,
   )
-  // In development, ensure placeholder CSS and CSS sourcemap files exist to avoid 404 noise
-  if (!isProduction) {
-    const chromiumCssPlaceholders = [
-      path.join(chromiumOutputDir, 'popup.css'),
-      path.join(chromiumOutputDir, 'content-script.css'),
-    ]
-    for (const p of chromiumCssPlaceholders) {
-      if (!(await fs.pathExists(p))) {
-        await fs.outputFile(p, '/* dev placeholder */\n')
-      }
-      const mapPath = `${p}.map`
-      if (!(await fs.pathExists(mapPath))) {
-        await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
-      }
-    }
-  }
+  await ensureDevCssPlaceholders(chromiumOutputDir)
   if (isProduction) await zipFolder(chromiumOutputDir)
 
   // firefox
@@ -544,22 +544,7 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
     [...commonFiles, { src: 'src/manifest.v2.json', dst: 'manifest.json' }],
     firefoxOutputDir,
   )
-  // In development, ensure placeholder CSS and CSS sourcemap files exist to avoid 404 noise
-  if (!isProduction) {
-    const firefoxCssPlaceholders = [
-      path.join(firefoxOutputDir, 'popup.css'),
-      path.join(firefoxOutputDir, 'content-script.css'),
-    ]
-    for (const p of firefoxCssPlaceholders) {
-      if (!(await fs.pathExists(p))) {
-        await fs.outputFile(p, '/* dev placeholder */\n')
-      }
-      const mapPath = `${p}.map`
-      if (!(await fs.pathExists(mapPath))) {
-        await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
-      }
-    }
-  }
+  await ensureDevCssPlaceholders(firefoxOutputDir)
   if (isProduction) await zipFolder(firefoxOutputDir)
 }