fix: enable browser executable selection

internal/platform/files/directory_picker.go

@@ -7,3 +7,10 @@ import "context"
 type DirectoryPicker interface {
 	ChooseDirectory(context.Context) (string, error)
 }
+
+// ExecutablePicker opens a platform file chooser for an existing browser
+// executable. The implementation must block only in its own goroutine; UI
+// callers receive the result asynchronously.
+type ExecutablePicker interface {
+	ChooseExecutable(context.Context) (string, error)
+}

internal/platform/files/executable_picker_other.go

@@ -0,0 +1,16 @@
+//go:build !windows
+
+package files
+
+import (
+	"context"
+	"errors"
+)
+
+type unsupportedExecutablePicker struct{}
+
+func NewExecutablePicker() ExecutablePicker { return unsupportedExecutablePicker{} }
+
+func (unsupportedExecutablePicker) ChooseExecutable(context.Context) (string, error) {
+	return "", errors.New("executable picker is only supported on Windows")
+}

internal/platform/files/executable_picker_windows.go

@@ -0,0 +1,54 @@
+//go:build windows
+
+package files
+
+import (
+	"context"
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+type windowsExecutablePicker struct{ runner commandRunner }
+
+func NewExecutablePicker() ExecutablePicker { return windowsExecutablePicker{runner: execRunner{}} }
+
+func newExecutablePickerWithRunner(runner commandRunner) windowsExecutablePicker {
+	return windowsExecutablePicker{runner: runner}
+}
+
+func (p windowsExecutablePicker) ChooseExecutable(ctx context.Context) (string, error) {
+	if err := ctx.Err(); err != nil {
+		return "", err
+	}
+	output, err := p.runner.Run(ctx, "powershell.exe",
+		"-NoProfile", "-NonInteractive", "-STA", "-WindowStyle", "Hidden", "-Command", executableDialogScript)
+	if err != nil {
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return "", ctxErr
+		}
+		return "", fmt.Errorf("open executable picker: %w", err)
+	}
+	selected := strings.TrimSpace(string(output))
+	if selected == "" {
+		return "", ErrSelectionCanceled
+	}
+	if !filepath.IsAbs(selected) {
+		return "", fmt.Errorf("executable picker returned a non-absolute path")
+	}
+	if !strings.EqualFold(filepath.Ext(selected), ".exe") {
+		return "", fmt.Errorf("executable picker returned a non-executable path")
+	}
+	return filepath.Clean(selected), nil
+}
+
+const executableDialogScript = `[Console]::OutputEncoding = [System.Text.UTF8Encoding]::new($false)
+Add-Type -AssemblyName System.Windows.Forms
+$dialog = New-Object System.Windows.Forms.OpenFileDialog
+$dialog.Title = '选择 Chrome 或 Edge 可执行文件'
+$dialog.Filter = '浏览器可执行文件 (*.exe)|*.exe|所有文件 (*.*)|*.*'
+$dialog.CheckFileExists = $true
+$dialog.Multiselect = $false
+if ($dialog.ShowDialog() -eq [System.Windows.Forms.DialogResult]::OK) {
+  [Console]::Out.Write($dialog.FileName)
+}`

internal/platform/files/executable_picker_windows_test.go

@@ -0,0 +1,51 @@
+//go:build windows
+
+package files
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestWindowsExecutablePickerReturnsCleanAbsoluteExecutable(t *testing.T) {
+	runner := &fakeRunner{out: []byte("C:\\Program Files\\Google\\Chrome\\Application\\chrome.exe\r\n")}
+	picker := newExecutablePickerWithRunner(runner)
+	got, err := picker.ChooseExecutable(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != `C:\Program Files\Google\Chrome\Application\chrome.exe` {
+		t.Fatalf("path = %q", got)
+	}
+	command := strings.Join(runner.args, " ")
+	if runner.name != "powershell.exe" || !strings.Contains(command, "-STA") || !strings.Contains(command, "OpenFileDialog") {
+		t.Fatalf("unexpected picker command: %s %v", runner.name, runner.args)
+	}
+}
+
+func TestWindowsExecutablePickerRejectsInvalidSelection(t *testing.T) {
+	tests := []struct {
+		name string
+		out  string
+	}{
+		{name: "relative", out: `chrome.exe`},
+		{name: "not executable", out: `C:\Browser\chrome.txt`},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			picker := newExecutablePickerWithRunner(&fakeRunner{out: []byte(test.out)})
+			if _, err := picker.ChooseExecutable(context.Background()); err == nil {
+				t.Fatal("invalid selection was accepted")
+			}
+		})
+	}
+}
+
+func TestWindowsExecutablePickerTreatsEmptySelectionAsCanceled(t *testing.T) {
+	picker := newExecutablePickerWithRunner(&fakeRunner{})
+	if _, err := picker.ChooseExecutable(context.Background()); !errors.Is(err, ErrSelectionCanceled) {
+		t.Fatalf("error = %v", err)
+	}
+}