feat: add --overlay-font-family option for caption font customization

Allow users to specify a custom font family for the overlay captions via
a new --overlay-font-family flag, defaulting to "Sans". Validation
ensures the font family is not empty. Updated README, config, and tests.

src/config/config.go

@@ -4,11 +4,13 @@ package config
 import (
 	"errors"
 	"flag"
+	"strings"
 	"time"
 )
 
 const (
 	DefaultModelsDir            = "models"
+	DefaultOverlayFontFamily    = "Sans"
 	MinimumOverlayFinalLifetime = 3 * time.Second
 )
 
@@ -60,6 +62,7 @@ type OverlaySettings struct {
 	Backend      OverlayBackend
 	Opacity      float64
 	FontSize     int
+	FontFamily   string
 	BottomMargin int
 	MaxWidth     int
 	Monitor      string
@@ -88,6 +91,7 @@ func ParseDesktop() DesktopSettings {
 	flags.StringVar(&backend, "overlay-backend", "auto", "display backend: auto, wayland, or x11")
 	flags.Float64Var(&desktop.Overlay.Opacity, "overlay-opacity", 0.90, "caption background opacity from 0 to 1")
 	flags.IntVar(&desktop.Overlay.FontSize, "overlay-font-size", 28, "caption font size in logical pixels")
+	flags.StringVar(&desktop.Overlay.FontFamily, "overlay-font-family", DefaultOverlayFontFamily, "caption font family")
 	flags.IntVar(&desktop.Overlay.BottomMargin, "overlay-bottom-margin", 100, "distance from the monitor bottom in logical pixels")
 	flags.IntVar(&desktop.Overlay.MaxWidth, "overlay-max-width", 1200, "fixed caption width in logical pixels before the monitor safety cap")
 	flags.StringVar(&desktop.Overlay.Monitor, "overlay-monitor", "auto", "monitor selection: auto, numeric index, or connector name")
@@ -129,6 +133,9 @@ func (s DesktopSettings) Validate() error {
 	if s.Overlay.FontSize <= 0 {
 		return errors.New("--overlay-font-size must be positive")
 	}
+	if strings.TrimSpace(s.Overlay.FontFamily) == "" {
+		return errors.New("--overlay-font-family cannot be empty")
+	}
 	if s.Overlay.BottomMargin < 0 {
 		return errors.New("--overlay-bottom-margin cannot be negative")
 	}