fix(hyprland): resolve desc: monitor names before generating config

Extract shared resolveMonitorNames to backend package so both
Hyprland and Sway backends resolve desc: prefixed names to actual
connector names before applying monitor layout.

Previously the Hyprland backend wrote desc:... literally into
monitors.conf, which Hyprland ignores, causing monitors to be
positioned incorrectly or not configured at all.

internal/backend/interface.go

@@ -8,7 +8,12 @@
 // cmd/monitor-lets-go/main.go.
 package backend
 
-import "context"
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"strings"
+)
 
 // MonitorInfo represents a physical display as reported by the compositor.
 // Fields use JSON tags matching hyprctl -j output; other compositors
@@ -97,3 +102,65 @@ type Backend interface {
 	// Close releases any resources held by the backend.
 	Close() error
 }
+
+// resolveMonitorNames converts desc: prefixed monitor config names to the
+// actual connector names (e.g. "desc:Xiaomi Corporation Mi Monitor" → "DP-9")
+// by matching against the current set of connected monitors.
+//
+// Matching logic:
+//   - "desc:make model" — substring match against description (make + " " + model)
+//   - "desc:serial" — substring match against serial number
+//   - plain names are returned as-is
+func resolveMonitorNames(ctx context.Context, monitors []MonitorConfig, getMonitors func(context.Context) ([]MonitorInfo, error), logger *slog.Logger) ([]MonitorConfig, error) {
+	var needsDescResolution bool
+	for _, m := range monitors {
+		if strings.HasPrefix(m.Name, "desc:") {
+			needsDescResolution = true
+			break
+		}
+	}
+	if !needsDescResolution {
+		return monitors, nil
+	}
+
+	current, err := getMonitors(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("get current monitors: %w", err)
+	}
+
+	resolved := make([]MonitorConfig, len(monitors))
+	for i, m := range monitors {
+		if !strings.HasPrefix(m.Name, "desc:") {
+			resolved[i] = m
+			continue
+		}
+
+		needle := strings.TrimPrefix(m.Name, "desc:")
+		var matches []MonitorInfo
+		for _, mi := range current {
+			if strings.Contains(mi.Description, needle) || strings.Contains(mi.Serial, needle) {
+				matches = append(matches, mi)
+			}
+		}
+
+		switch len(matches) {
+		case 0:
+			return nil, fmt.Errorf("no connected monitor matches desc:%q", needle)
+		case 1:
+			resolved[i] = m
+			resolved[i].Name = matches[0].Name
+			logger.Debug("resolved desc to connector",
+				"desc", needle, "connector", matches[0].Name)
+		default:
+			var names []string
+			for _, mat := range matches {
+				names = append(names, mat.Name)
+			}
+			return nil, fmt.Errorf(
+				"desc:%q matches multiple monitors: %s — use a more specific identifier",
+				needle, strings.Join(names, ", "))
+		}
+	}
+
+	return resolved, nil
+}