fix(tui): don't use path/filepath for embed

EmbedFS uses '/' always for file paths. It does not support '\' even on
Windows. Make sure we don't give EmbedFS a '\' path on Windows. This
lets the TUI find the theme files, avoiding crashes in various parts of
the code that assume a theme exists.

packages/tui/internal/theme/loader.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"image/color"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 
@@ -42,7 +43,7 @@ func LoadThemesFromJSON() error {
 			continue
 		}
 		themeName := strings.TrimSuffix(entry.Name(), ".json")
-		data, err := themesFS.ReadFile(filepath.Join("themes", entry.Name()))
+		data, err := themesFS.ReadFile(path.Join("themes", entry.Name()))
 		if err != nil {
 			return fmt.Errorf("failed to read theme file %s: %w", entry.Name(), err)
 		}