feat: add CUE sheet support for local library scanning and splitting (#201)

Parse .cue files in library scanner (Go + SAF) to display individual
tracks instead of one large audio file. Add FFmpeg-based CUE splitting
to extract tracks into separate FLAC files with embedded metadata and
cover art.

- Go: CUE parser, two-pass scan (CUE first, skip referenced audio),
  virtual paths (cue#trackNN) for DB UNIQUE constraint, audioDir
  override for SAF temp-file scenarios
- Android: SAF scanner recognizes .cue in both full and incremental
  scan, copies .cue+audio to temp for Go parsing, unchanged-CUE audio
  sibling dedup, parseCueSheet handler resolves SAF audio siblings
- Dart: FFmpegService.splitCueToTracks, CUE split UI in track metadata
  screen, persistent output dir for SAF splits with write-back
- CUE virtual path normalization across fileExists/fileStat/deleteFile/
  openFile; play/share/open blocked for virtual tracks with guidance to
  split first; delete only removes DB entry, not shared .cue file
- iOS: parseCueSheet handler
- Localization: 12 new CUE-related strings

Requested by @Seerafimm
Closes #201

ios/Runner/AppDelegate.swift

@@ -969,6 +969,15 @@ import Gobackend  // Import Go framework
             if let error = error { throw error }
             return response
             
+        // CUE Sheet Parsing
+        case "parseCueSheet":
+            let args = call.arguments as! [String: Any]
+            let cuePath = args["cue_path"] as! String
+            let audioDir = args["audio_dir"] as? String ?? ""
+            let response = GobackendParseCueSheet(cuePath, audioDir, &error)
+            if let error = error { throw error }
+            return response
+            
         default:
             throw NSError(
                 domain: "SpotiFLAC",