test: cron schedule + webhook URL + canvas layout coverage (#73)

- calculateNextRun: field-count, garbage, bad-zone errors; CRON_TZ
  actually shifts the instant (UTC vs Asia/Tokyo)
- validWebhookURL: http/https only, host required, whitespace trimmed
- canvas layout math extracted to canvas/layout.ts (component files may
  only export components) and covered: rectsOverlap gap zone,
  resolveFreeSpot grid-snap/collision/bounds-fallback, groupSlot
  fit/append/grow/ignore-dragged, buildMetadataFromFlow serialization

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/cron_test.go

@@ -0,0 +1,66 @@
+package api
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestCalculateNextRun(t *testing.T) {
+	cases := []struct {
+		name     string
+		schedule string
+		timezone string
+		wantErr  bool
+	}{
+		{"every minute", "* * * * *", "", false},
+		{"daily at 4am", "0 4 * * *", "UTC", false},
+		{"weekday run", "0 9 * * 1-5", "Europe/Prague", false},
+		{"whitespace tolerated", "  */15 * * * *  ", "", false},
+		{"empty schedule", "", "", true},
+		{"bad field count", "* * *", "", true},
+		{"garbage", "not a cron", "", true},
+		{"bad timezone", "0 4 * * *", "Mars/Olympus", true},
+	}
+	now := time.Now()
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			next, err := calculateNextRun(tc.schedule, tc.timezone)
+			if tc.wantErr {
+				if err == nil {
+					t.Fatalf("expected error, got next=%v", next)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if next == nil {
+				t.Fatal("expected non-nil next run")
+			}
+			if !next.After(now) {
+				t.Errorf("next run %v is not after %v", next, now)
+			}
+		})
+	}
+}
+
+func TestCalculateNextRunTimezone(t *testing.T) {
+	// CRON_TZ must change the computed instant: 04:00 in a fixed-offset zone
+	// lands at a different UTC hour than 04:00 UTC.
+	utcNext, err := calculateNextRun("0 4 * * *", "UTC")
+	if err != nil {
+		t.Fatalf("utc: %v", err)
+	}
+	tokyoNext, err := calculateNextRun("0 4 * * *", "Asia/Tokyo")
+	if err != nil {
+		t.Fatalf("tokyo: %v", err)
+	}
+	if utcNext.Equal(*tokyoNext) {
+		t.Fatalf("timezone had no effect: both %v", utcNext)
+	}
+	if !strings.EqualFold(utcNext.Location().String(), "Local") {
+		// UTC-scheduled runs still land in server-local time representation.
+		t.Logf("utc next: %v / tokyo next: %v", utcNext, tokyoNext)
+	}
+}