feat: allow overriding the output directory via GODEEZ_OUTPUT_DIR

The output directory used to be fixed at ~/Music/GoDeez. config.Load
now honours GODEEZ_OUTPUT_DIR when set, with a leading ~ expanded by
hand since the shell only does that for unquoted arguments, not for
values read out of the environment. Falls back to the previous default
when unset.

internal/config/config.go

@@ -1,6 +1,7 @@
 // Package config resolves where godeez reads its session from and writes its
-// downloads to. There is no config file: the output directory is fixed and
-// the only setting is the DEEZER_ARL environment variable, which exists as an
+// downloads to. There is no config file: the output directory defaults to
+// ~/Music/GoDeez but can be overridden with GODEEZ_OUTPUT_DIR, and the only
+// other setting is the DEEZER_ARL environment variable, which exists as an
 // escape hatch for users who would rather not store credentials in the system
 // keyring.
 package config
@@ -9,6 +10,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/mathismqn/godeez/internal/fsutil"
 )
@@ -27,12 +29,11 @@ type Config struct {
 func Load() (*Config, error) {
 	arl := os.Getenv("DEEZER_ARL")
 
-	homeDir, err := os.UserHomeDir()
+	outputDir, err := resolveOutputDir()
 	if err != nil {
-		return nil, fmt.Errorf("failed to get home directory: %w", err)
+		return nil, err
 	}
 
-	outputDir := filepath.Join(homeDir, "Music", "GoDeez")
 	if err := fsutil.EnsureDir(outputDir); err != nil {
 		return nil, fmt.Errorf("failed to create output directory: %w", err)
 	}
@@ -42,3 +43,29 @@ func Load() (*Config, error) {
 		OutputDir: outputDir,
 	}, nil
 }
+
+// resolveOutputDir honours GODEEZ_OUTPUT_DIR when set, falling back to
+// ~/Music/GoDeez otherwise.
+//
+// A leading "~" is expanded by hand because the shell only does that for
+// unquoted arguments, not for values read out of the environment, so users
+// setting this in a profile file would otherwise end up with a literal "~"
+// directory.
+func resolveOutputDir() (string, error) {
+	homeDir, err := os.UserHomeDir()
+	if err != nil {
+		return "", fmt.Errorf("failed to get home directory: %w", err)
+	}
+
+	if dir := strings.TrimSpace(os.Getenv("GODEEZ_OUTPUT_DIR")); dir != "" {
+		if dir == "~" {
+			return homeDir, nil
+		}
+		if rest, ok := strings.CutPrefix(dir, "~/"); ok {
+			return filepath.Join(homeDir, rest), nil
+		}
+		return dir, nil
+	}
+
+	return filepath.Join(homeDir, "Music", "GoDeez"), nil
+}