fix(installer): replace orphaned neofetch with fastfetch, surface stderr on Run failure

- neofetch was removed from Arch's official repos; fastfetch is the maintained replacement.
- ExecRunner.Run previously discarded stderr, so any 'sudo pacman' failure surfaced only as 'exit status 1'. Now the returned error includes the captured stderr (matching what Output already did).

internal/config/defaults.yaml

@@ -13,7 +13,7 @@ packages:
     - zip
     - tree
     - htop
-    - neofetch
+    - fastfetch
     - ripgrep
     - fd
     - bat

internal/platform/pacman.go

@@ -14,9 +14,17 @@ type ExecRunner struct{}
 
 func (ExecRunner) Run(ctx context.Context, name string, args ...string) error {
 	cmd := exec.CommandContext(ctx, name, args...)
+	var stderr bytes.Buffer
 	cmd.Stdout = nil
-	cmd.Stderr = nil
+	cmd.Stderr = &stderr
-	return cmd.Run()
+	if err := cmd.Run(); err != nil {
+		msg := strings.TrimSpace(stderr.String())
+		if msg == "" {
+			return fmt.Errorf("%s %s: %w", name, strings.Join(args, " "), err)
+		}
+		return fmt.Errorf("%s %s: %w: %s", name, strings.Join(args, " "), err, msg)
+	}
+	return nil
 }
 
 func (ExecRunner) Output(ctx context.Context, name string, args ...string) ([]byte, error) {