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")
 	}

src/config/config_test.go

@@ -20,7 +20,7 @@ func validDesktopSettings() DesktopSettings {
 		Settings: Settings{Mode: ModeVAD, ChunkDuration: time.Second, Threads: 2},
 		Output:   OutputOverlay,
 		Overlay: OverlaySettings{
-			Backend: BackendAuto, Opacity: 0.9, FontSize: 28, BottomMargin: 100,
+			Backend: BackendAuto, Opacity: 0.9, FontSize: 28, FontFamily: DefaultOverlayFontFamily, BottomMargin: 100,
 			MaxWidth: 1200, Monitor: "auto", FinalTimeout: 4 * time.Second, ClickThrough: true,
 		},
 	}
@@ -40,6 +40,7 @@ func TestDesktopSettingsValidate(t *testing.T) {
 		{"negative opacity", func(s *DesktopSettings) { s.Overlay.Opacity = -0.1 }},
 		{"large opacity", func(s *DesktopSettings) { s.Overlay.Opacity = 1.1 }},
 		{"zero font", func(s *DesktopSettings) { s.Overlay.FontSize = 0 }},
+		{"empty font family", func(s *DesktopSettings) { s.Overlay.FontFamily = "  " }},
 		{"negative margin", func(s *DesktopSettings) { s.Overlay.BottomMargin = -1 }},
 		{"zero width", func(s *DesktopSettings) { s.Overlay.MaxWidth = 0 }},
 		{"empty monitor", func(s *DesktopSettings) { s.Overlay.Monitor = "" }},

src/output/overlay/overlay_gtk.go

@@ -246,10 +246,27 @@ static gboolean captioneer_window_close(GtkWindow *window, gpointer data) {
 	return TRUE;
 }
 
+static char *captioneer_font_family_rule(const char *font_family) {
+	GString *rule = g_string_new("font-family: \"");
+	for (const char *character = font_family; *character != '\0'; character++) {
+		if (*character == '\\' || *character == '\"') {
+			g_string_append_c(rule, '\\');
+		}
+		if (*character == '\n' || *character == '\r' || *character == '\t') {
+			g_string_append_c(rule, ' ');
+		} else {
+			g_string_append_c(rule, *character);
+		}
+	}
+	g_string_append(rule, "\";");
+	return g_string_free(rule, FALSE);
+}
+
 static CaptioneerOverlay *captioneer_overlay_new(
 	const char *backend,
 	double opacity,
 	int font_size,
+	const char *font_family,
 	int bottom_margin,
 	int max_width,
 	const char *monitor_selection,
@@ -372,17 +389,19 @@ static CaptioneerOverlay *captioneer_overlay_new(
 	gtk_widget_add_css_class(overlay->preview_label, "captioneer-preview");
 	gtk_window_set_child(overlay->window, box);
 
+	char *font_family_rule = captioneer_font_family_rule(font_family);
 	char *css = g_strdup_printf(
 		"window.captioneer-overlay { background-color: transparent; box-shadow: none; }"
 		".captioneer-bubble { background-color: rgba(0, 0, 0, %.3f); border-radius: 12px; padding: 14px 22px; }"
-		".captioneer-final { color: rgba(255, 255, 255, 1.0); font-size: %dpx; }"
-		".captioneer-preview { color: rgba(255, 255, 255, 0.78); font-size: %dpx; }",
-		opacity, font_size, font_size);
+		".captioneer-final { color: rgba(255, 255, 255, 1.0); font-size: %dpx; %s }"
+		".captioneer-preview { color: rgba(255, 255, 255, 0.78); font-size: %dpx; %s }",
+		opacity, font_size, font_family_rule, font_size, font_family_rule);
 	GtkCssProvider *provider = gtk_css_provider_new();
 	gtk_css_provider_load_from_string(provider, css);
 	gtk_style_context_add_provider_for_display(display, GTK_STYLE_PROVIDER(provider), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
 	g_object_unref(provider);
 	g_free(css);
+	g_free(font_family_rule);
 
 	gtk_widget_realize(GTK_WIDGET(overlay->window));
 	GdkSurface *surface = gtk_native_get_surface(GTK_NATIVE(overlay->window));
@@ -465,8 +484,10 @@ type Sink struct {
 
 func New(settings config.OverlaySettings) (*Sink, string, error) {
 	backend := C.CString(string(settings.Backend))
+	fontFamily := C.CString(settings.FontFamily)
 	monitor := C.CString(settings.Monitor)
 	defer C.free(unsafe.Pointer(backend))
+	defer C.free(unsafe.Pointer(fontFamily))
 	defer C.free(unsafe.Pointer(monitor))
 
 	var warningMessage *C.char
@@ -475,6 +496,7 @@ func New(settings config.OverlaySettings) (*Sink, string, error) {
 		backend,
 		C.double(settings.Opacity),
 		C.int(settings.FontSize),
+		fontFamily,
 		C.int(settings.BottomMargin),
 		C.int(settings.MaxWidth),
 		monitor,