Add CSS sourcemap placeholders for dev outputs

Avoid 404s on *.css.map when using external source maps by generating
minimal placeholder maps alongside popup.css and content-script.css for
Chromium and Firefox dev builds. No impact on production artifacts

Author: PeterDaveHello

GEMINI.md

@@ -0,0 +1 @@
+AGENTS.md

build.mjs

@@ -501,7 +501,7 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
     [...commonFiles, { src: 'src/manifest.json', dst: 'manifest.json' }],
     chromiumOutputDir,
   )
-  // In development, ensure placeholder CSS files exist to avoid 404 noise
+  // In development, ensure placeholder CSS and CSS sourcemap files exist to avoid 404 noise
   if (!isProduction) {
     const chromiumCssPlaceholders = [
       path.join(chromiumOutputDir, 'popup.css'),
@@ -511,6 +511,10 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
       if (!(await fs.pathExists(p))) {
         await fs.outputFile(p, '/* dev placeholder */\n')
       }
+      const mapPath = `${p}.map`
+      if (!(await fs.pathExists(mapPath))) {
+        await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
+      }
     }
   }
   if (isProduction) await zipFolder(chromiumOutputDir)
@@ -521,7 +525,7 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
     [...commonFiles, { src: 'src/manifest.v2.json', dst: 'manifest.json' }],
     firefoxOutputDir,
   )
-  // In development, ensure placeholder CSS files exist to avoid 404 noise
+  // In development, ensure placeholder CSS and CSS sourcemap files exist to avoid 404 noise
   if (!isProduction) {
     const firefoxCssPlaceholders = [
       path.join(firefoxOutputDir, 'popup.css'),
@@ -531,6 +535,10 @@ async function finishOutput(outputDirSuffix, sourceBuildDir = outdir) {
       if (!(await fs.pathExists(p))) {
         await fs.outputFile(p, '/* dev placeholder */\n')
       }
+      const mapPath = `${p}.map`
+      if (!(await fs.pathExists(mapPath))) {
+        await fs.outputFile(mapPath, '{"version":3,"sources":[],"mappings":"","names":[]}')
+      }
     }
   }
   if (isProduction) await zipFolder(firefoxOutputDir)