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.go

@@ -0,0 +1,79 @@
+// Monitor enumeration for the settings UI.
+//
+// The screen-capture backend itself selects a monitor by index; this file
+// provides a way to list the available monitors so the GUI can present a
+// friendly picker instead of a raw index.
+//
+// Hyprland exposes monitor info via the `hyprctl monitors` command. Other
+// Wayland compositors would need a portal-based enumeration; for now we only
+// implement the Hyprland path (the primary dev environment) and return a
+// clear error elsewhere.
+
+package capture
+
+import (
+	"encoding/json"
+	"errors"
+	"os/exec"
+)
+
+// Monitor describes one capturable output.
+type Monitor struct {
+	Index   int    // capture StreamIndex to pass to OpenPipeWire
+	Name    string // compositor name, e.g. "eDP-1"
+	Width   int
+	Height  int
+	Primary bool
+}
+
+// ErrNoMonitors is returned when monitor enumeration is unsupported or fails.
+var ErrNoMonitors = errors.New("capture: monitor enumeration unavailable on this compositor")
+
+// ListMonitors returns the available monitors for the GUI picker.
+func ListMonitors() ([]Monitor, error) {
+	if hyprctlAvailable() {
+		return hyprctlMonitors()
+	}
+	return nil, ErrNoMonitors
+}
+
+// hyprctlAvailable reports whether the Hyprland monitor command exists.
+func hyprctlAvailable() bool {
+	_, err := exec.LookPath("hyprctl")
+	return err == nil
+}
+
+// hyprctlMonitor is the JSON shape emitted by `hyprctl monitors -j`.
+type hyprctlMonitor struct {
+	ID          int    `json:"id"`
+	Name        string `json:"name"`
+	Width       int    `json:"width"`
+	Height      int    `json:"height"`
+	Description string `json:"description"`
+	Focused     bool   `json:"focused"`
+}
+
+// hyprctlMonitors lists monitors via the Hyprland IPC command.
+func hyprctlMonitors() ([]Monitor, error) {
+	out, err := exec.Command("hyprctl", "monitors", "-j").Output()
+	if err != nil {
+		return nil, ErrNoMonitors
+	}
+
+	var raw []hyprctlMonitor
+	if err := json.Unmarshal(out, &raw); err != nil {
+		return nil, ErrNoMonitors
+	}
+
+	monitors := make([]Monitor, 0, len(raw))
+	for _, m := range raw {
+		monitors = append(monitors, Monitor{
+			Index:   m.ID,
+			Name:    m.Name,
+			Width:   m.Width,
+			Height:  m.Height,
+			Primary: m.Focused,
+		})
+	}
+	return monitors, nil
+}