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/config/config.go

@@ -16,13 +16,14 @@ import (
 
 // Config mirrors flinger.Config with JSON tags for persistence.
 type Config struct {
-	Name        string `json:"name"`
-	Port        int    `json:"port"`
-	Quality     int    `json:"quality"`
-	FPS         int    `json:"fps"`
-	Source      string `json:"source"`
-	Audio       bool   `json:"audio"`
-	StreamIndex int    `json:"stream_index"`
+	Name        string  `json:"name"`
+	Port        int     `json:"port"`
+	Quality     int     `json:"quality"`
+	FPS         int     `json:"fps"`
+	Source      string  `json:"source"`
+	Audio       bool    `json:"audio"`
+	StreamIndex int     `json:"stream_index"`
+	Scale       float64 `json:"scale"`
 	// Announce is a *bool so an absent JSON key (older config files) keeps
 	// the default instead of silently disabling announcements.
 	Announce *bool `json:"announce"`
@@ -40,6 +41,7 @@ func Default() Config {
 		Source:      c.Source,
 		Audio:       c.Audio,
 		StreamIndex: c.StreamIndex,
+		Scale:       c.Scale,
 		Announce:    &announce,
 	}
 }
@@ -58,6 +60,7 @@ func (c Config) ToFlinger() flinger.Config {
 		Source:      c.Source,
 		Audio:       c.Audio,
 		StreamIndex: c.StreamIndex,
+		Scale:       c.Scale,
 		Announce:    announce,
 	}
 }
@@ -111,6 +114,9 @@ func LoadFrom(path string) (Config, error) {
 	if c.Source == "" {
 		c.Source = d.Source
 	}
+	if c.Scale <= 0 || c.Scale > 1 {
+		c.Scale = d.Scale
+	}
 	if c.Announce == nil {
 		announce := true
 		c.Announce = &announce