fix: resolve all lint errors and make linting a required CI check (#62)

- Fix 10 err113 violations: extract dynamic errors to package-level sentinels
- Fix 12 errcheck violations: handle unchecked error returns in player,
  metadata, and config packages
- Fix 4 revive stutter warnings: rename player.PlayerState to player.State,
  player.PlayerVolume to player.Volume, queue.QueueTrack to queue.Track,
  queue.QueueState to queue.State
- Fix 2 staticcheck SA4001: simplify *&x to x in assets handler
- Fix 5 unused constants: remove dead AudioFileType iota block in models
- Fix gci/gofumpt/wsl formatting issues across multiple files
- Add gofumpt module-path setting to .golangci.yml for correct import grouping
- Fix player test: gate integration test behind YELLOWJACKET_INTEGRATION env var
  instead of only skipping in CI, and replace t.Errorf+t.Failed with t.Fatalf
- Remove continue-on-error from golangci-lint CI step so linting is now required

backend/library/library.go

@@ -17,12 +17,15 @@ import (
 
 	"github.com/wailsapp/wails/v2/pkg/runtime"
 	"golang.org/x/sync/errgroup"
+
 	"yellowjacket/backend/database"
 	"yellowjacket/backend/database/sql/sqlcgen"
 	"yellowjacket/backend/events"
 	"yellowjacket/backend/metadata"
 )
 
+var errLibraryDirNotConfigured = errors.New("library directory not configured")
+
 // Library manages scanning and querying the music collection.
 type Library struct {
 	ctx    context.Context
@@ -107,7 +110,7 @@ func (l *Library) Scan() error {
 	l.logger.Info("beginning library scan", "workers", scanWorkerCount)
 
 	if len(l.conf.DirectoryPath) == 0 {
-		return errors.New("library directory not configured")
+		return errLibraryDirNotConfigured
 	}
 
 	// Load existing file paths from the database into a sync.Map for concurrent access.