feat(learn): add audio paths for all courses, convert TTS to MP3, fix generator script

- Add audioPath for 3 legacy courses (Basics of Prayer, Understanding Fiqh, Quranic Arabic)
- Convert new course TTS from M4A to MP3 for consistency
- Fix generate-tts-local.js ext replacement bug (works with .mp3 now)
- All 7 courses now have 32 MP3 modules total (~74MB)
- Build passes (0 errors, 0 warnings)

scripts/generate-tts-local.js

@@ -46,7 +46,7 @@ function stripMarkdown(text) {
 }
 
 function generateSayM4a(text, outputPath) {
-  const tmpTxt = outputPath.replace('.m4a', '.txt');
+  const tmpTxt = outputPath.replace(path.extname(outputPath), '.txt');
   fs.writeFileSync(tmpTxt, text, 'utf-8');
   try {
     // macOS say outputs AAC in .m4a directly with --file-format m4af