feat: screen capture scaling / downsampling

Add a Scale option (1.0 native, 0.5 half, etc.) that downsamples the
captured BGRA frame with bilinear interpolation before JPEG encoding.
Config carries the scale factor; the GUI exposes a Scale dropdown
(100%/75%/50%/25%) that applies live via SetConfig.

Verified: OBS displays the image at the reduced resolution matching the
selected scale, without restarting the stream.

internal/gui/gui.go

@@ -36,6 +36,34 @@ func itoa(v int) string { return strconv.Itoa(v) }
 
 func atoi(s string) (int, error) { return strconv.Atoi(s) }
 
+// scaleLabel renders a scale factor as a percentage option.
+func scaleLabel(scale float64) string {
+	switch {
+	case scale >= 0.95:
+		return "100%"
+	case scale >= 0.7:
+		return "75%"
+	case scale >= 0.45:
+		return "50%"
+	default:
+		return "25%"
+	}
+}
+
+// parseScale converts a scale option label back to a factor.
+func parseScale(label string) float64 {
+	switch label {
+	case "75%":
+		return 0.75
+	case "50%":
+		return 0.5
+	case "25%":
+		return 0.25
+	default:
+		return 1.0
+	}
+}
+
 // appID is the Fyne application ID used for preferences/settings storage.
 const appID = "io.teleportfling"
 
@@ -62,6 +90,7 @@ type App struct {
 	qualitySel  *widget.Select
 	fpsSel      *widget.Select
 	presetSel   *widget.Select
+	scaleSel    *widget.Select
 	nameEnt     *widget.Entry
 	audioChk    *widget.Check
 	announceChk *widget.Check
@@ -148,6 +177,10 @@ func (g *App) buildUI() {
 	g.fpsSel = widget.NewSelect([]string{"15", "30", "60"}, g.applyLiveSettings)
 	g.fpsSel.SetSelected(itoa(g.cfg.FPS))
 
+	// Scale: downsampling factor (1.0 = native, 0.5 = half, etc).
+	g.scaleSel = widget.NewSelect([]string{"100%", "75%", "50%", "25%"}, g.applyLiveSettings)
+	g.scaleSel.SetSelected(scaleLabel(g.cfg.Scale))
+
 	// Preset: one-click quality/fps combos. Choosing one sets the Quality and
 	// Frame rate selectors and applies them (live if running). Created after
 	// the quality/fps selects so applyPreset's references are valid.
@@ -183,6 +216,7 @@ func (g *App) buildUI() {
 			{Text: "Preset", Widget: g.presetSel},
 			{Text: "Quality", Widget: g.qualitySel},
 			{Text: "Frame rate", Widget: g.fpsSel},
+			{Text: "Scale", Widget: g.scaleSel},
 			{Text: "", Widget: g.audioChk},
 			{Text: "", Widget: g.announceChk},
 		},
@@ -287,6 +321,7 @@ func (g *App) applyLiveSettings(string) {
 		Source:      g.cfg.Source,
 		Audio:       g.audioChk.Checked,
 		StreamIndex: g.cfg.StreamIndex,
+		Scale:       parseScale(g.scaleSel.Selected),
 		Announce:    announce,
 	}
 	if err := g.eng.SetConfig(cfg); err != nil {
@@ -298,6 +333,7 @@ func (g *App) applyLiveSettings(string) {
 	g.cfg.FPS = fps
 	g.cfg.Audio = cfg.Audio
 	g.cfg.Name = cfg.Name
+	g.cfg.Scale = cfg.Scale
 	ann := announce
 	g.cfg.Announce = &ann
 }
@@ -329,6 +365,7 @@ func (g *App) start() {
 		Source:      g.srcSel.Selected,
 		Audio:       g.audioChk.Checked,
 		StreamIndex: g.selectedMonitorIndex(),
+		Scale:       parseScale(g.scaleSel.Selected),
 		Announce:    &announce,
 	}
 	if g.configPath != "" {