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/config.go

@@ -2,10 +2,13 @@
 package library
 
 import (
+	"errors"
 	"fmt"
 	"os"
 )
 
+var errNotDirectory = errors.New("path is not a directory")
+
 // Config holds Library config data.
 type Config struct {
 	DirectoryPath Directory `form:"Directory" schema:"directory,required"`
@@ -35,7 +38,7 @@ func (c *Config) Validate() error {
 		}
 
 		if !dirInfo.IsDir() {
-			return fmt.Errorf("%s is not a directory", c.DirectoryPath)
+			return fmt.Errorf("%s: %w", c.DirectoryPath, errNotDirectory)
 		}
 	}