Use env variable for max file size limit

- Frontend: read VITE_MAX_FILE_SIZE_MB from env
- Update .env.example files with limits

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/.env.example

@@ -1,2 +1,3 @@
 VITE_API_URL=http://localhost:4001
 VITE_WS_URL=ws://localhost:4001
+VITE_MAX_FILE_SIZE_MB=10

frontend/src/components/tracks/UploadTrack.vue

@@ -1,7 +1,7 @@
 <template>
   <form @submit.prevent="handleUpload" class="upload-form">
     <div class="form-group">
-      <label>MP3 файл (макс. 10MB)</label>
+      <label>MP3 файл (макс. {{ maxFileSizeMb }}MB)</label>
       <input
         type="file"
         accept="audio/mpeg,audio/mp3"
@@ -34,6 +34,9 @@ const emit = defineEmits(['uploaded'])
 
 const tracksStore = useTracksStore()
 
+const maxFileSizeMb = import.meta.env.VITE_MAX_FILE_SIZE_MB || 10
+const maxFileSize = maxFileSizeMb * 1024 * 1024
+
 const title = ref('')
 const artist = ref('')
 const file = ref(null)
@@ -45,9 +48,9 @@ function handleFileSelect(e) {
   const selectedFile = e.target.files[0]
   if (!selectedFile) return
 
-  // Check file size (10MB)
-  if (selectedFile.size > 10 * 1024 * 1024) {
-    error.value = 'Файл слишком большой (макс. 10MB)'
+  // Check file size
+  if (selectedFile.size > maxFileSize) {
+    error.value = `Файл слишком большой (макс. ${maxFileSizeMb}MB)`
     fileInput.value.value = ''
     return
   }