feat: add local library scanning with duplicate detection

- Add Go backend library scanner for FLAC, M4A, MP3, Opus, OGG files
- Read metadata from file tags (ISRC, track name, artist, album, bit depth, sample rate)
- Fallback to filename parsing when tags unavailable
- Add SQLite database for O(1) duplicate lookups
- Show 'In Library' badge on search results for existing tracks
- Match by ISRC (exact) or track name + artist (fuzzy)
- Add Library Settings page with scan, cleanup, and clear actions
- Add 30+ localization strings for library feature

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -892,6 +892,33 @@ class MainActivity: FlutterActivity() {
                             }
                             result.success(response)
                         }
+                        // Local Library Scanning
+                        "scanLibraryFolder" -> {
+                            val folderPath = call.argument<String>("folder_path") ?: ""
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.scanLibraryFolderJSON(folderPath)
+                            }
+                            result.success(response)
+                        }
+                        "getLibraryScanProgress" -> {
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.getLibraryScanProgressJSON()
+                            }
+                            result.success(response)
+                        }
+                        "cancelLibraryScan" -> {
+                            withContext(Dispatchers.IO) {
+                                Gobackend.cancelLibraryScanJSON()
+                            }
+                            result.success(null)
+                        }
+                        "readAudioMetadata" -> {
+                            val filePath = call.argument<String>("file_path") ?: ""
+                            val response = withContext(Dispatchers.IO) {
+                                Gobackend.readAudioMetadataJSON(filePath)
+                            }
+                            result.success(response)
+                        }
                         else -> result.notImplemented()
                     }
                 } catch (e: Exception) {