feat: add monitor picker to the GUI settings

Enumerate monitors via hyprctl (Hyprland) and present them as a
dropdown in the settings window instead of a raw index. The picker is
enabled only for the screen source and persists the selected monitor as
stream_index. Falls back to a single indexed option when enumeration is
unavailable (non-Hyprland).

internal/capture/monitors_test.go

@@ -0,0 +1,42 @@
+package capture
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+// TestHyprctlMonitors parses a representative `hyprctl monitors -j` payload.
+func TestHyprctlMonitors(t *testing.T) {
+	payload := `[
+  {
+    "id": 0,
+    "name": "eDP-1",
+    "description": "BOE 0x094C",
+    "width": 1920,
+    "height": 1200,
+    "focused": true
+  },
+  {
+    "id": 1,
+    "name": "HDMI-A-1",
+    "description": "Samsung",
+    "width": 2560,
+    "height": 1440,
+    "focused": false
+  }
+]`
+
+	var raw []hyprctlMonitor
+	if err := json.Unmarshal([]byte(payload), &raw); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if len(raw) != 2 {
+		t.Fatalf("got %d monitors, want 2", len(raw))
+	}
+	if raw[0].ID != 0 || raw[0].Name != "eDP-1" || raw[0].Width != 1920 {
+		t.Errorf("monitor 0 wrong: %+v", raw[0])
+	}
+	if raw[1].ID != 1 || raw[1].Focused {
+		t.Errorf("monitor 1 wrong: %+v", raw[1])
+	}
+}