refactor(cron): use UTC as default timezone, log invalid TZ warning

Address reviewer feedback (yinwm):
- Change hard-coded default from Asia/Shanghai to UTC, which is the
  industry standard for open-source projects (Kubernetes, AWS, etc.)
- Add warning log when time.LoadLocation fails for a configured timezone
  to help users debug misconfiguration
- Update tests to reflect UTC default

pkg/config/config.go

@@ -580,7 +580,7 @@ type WebToolsConfig struct {
 type CronToolsConfig struct {
 	ToolConfig         `    envPrefix:"PICOCLAW_TOOLS_CRON_"`
 	ExecTimeoutMinutes int    `                                 env:"PICOCLAW_TOOLS_CRON_EXEC_TIMEOUT_MINUTES" json:"exec_timeout_minutes"` // 0 means no timeout
-	DefaultTimezone    string `                                 env:"PICOCLAW_TOOLS_CRON_DEFAULT_TIMEZONE"      json:"default_timezone"`     // default timezone for cron expressions, e.g. "UTC"
+	DefaultTimezone    string `                                 env:"PICOCLAW_TOOLS_CRON_DEFAULT_TIMEZONE"      json:"default_timezone"`    // default timezone for cron expressions, e.g. "UTC"
 }
 
 type ExecConfig struct {

pkg/cron/service.go

@@ -267,16 +267,18 @@ func (cs *CronService) computeNextRun(schedule *CronSchedule, nowMS int64) *int6
 
 		// Use gronx to calculate next run time
 		now := time.UnixMilli(nowMS)
-		// Apply timezone: schedule.TZ > service default > "Asia/Shanghai"
+		// Apply timezone: schedule.TZ > service default > UTC
 		tz := schedule.TZ
 		if tz == "" {
 			tz = cs.defaultTZ
 		}
 		if tz == "" {
-			tz = "Asia/Shanghai"
+			tz = "UTC"
 		}
 		if loc, err := time.LoadLocation(tz); err == nil {
 			now = now.In(loc)
+		} else {
+			log.Printf("[cron] warning: failed to load timezone '%s': %v, using UTC", tz, err)
 		}
 		nextTime, err := gronx.NextTickAfter(schedule.Expr, now, false)
 		if err != nil {
@@ -326,7 +328,7 @@ func (cs *CronService) SetOnJob(handler JobHandler) {
 }
 
 // SetDefaultTimezone sets the default timezone for cron expressions.
-// If empty, falls back to "Asia/Shanghai".
+// If empty, falls back to UTC.
 func (cs *CronService) SetDefaultTimezone(tz string) {
 	cs.mu.Lock()
 	defer cs.mu.Unlock()

pkg/cron/timezone_test.go

@@ -33,9 +33,9 @@ func TestComputeNextRun_CronTimezone(t *testing.T) {
 			wantHour: 1, // 9:00 CST = 1:00 UTC
 		},
 		{
-			name:     "empty TZ defaults to Asia/Shanghai",
+			name:     "empty TZ defaults to UTC",
 			tz:       "",
-			wantHour: 1, // should default to Asia/Shanghai
+			wantHour: 9, // should default to UTC
 		},
 		{
 			name:     "US/Eastern timezone",
@@ -73,13 +73,16 @@ func TestComputeNextRun_CronTimezone(t *testing.T) {
 	}
 }
 
-// TestComputeNextRun_DefaultTZ_NotUTC verifies that when TZ is empty,
+// TestComputeNextRun_DefaultTZ_AppliesWhenSet verifies that when TZ is empty
-// the computed next run differs from a pure UTC computation.
+// and no service default is set, the computation uses UTC.
-func TestComputeNextRun_DefaultTZ_NotUTC(t *testing.T) {
+func TestComputeNextRun_DefaultTZ_AppliesWhenSet(t *testing.T) {
 	tmpDir := t.TempDir()
 	storePath := filepath.Join(tmpDir, "jobs.json")
 	cs := NewCronService(storePath, nil)
 
+	// Set a non-UTC default to verify it takes effect
+	cs.SetDefaultTimezone("Asia/Shanghai")
+
 	now := time.Date(2026, 3, 4, 2, 0, 0, 0, time.UTC) // 02:00 UTC = 10:00 CST
 	nowMS := now.UnixMilli()
 
@@ -93,12 +96,12 @@ func TestComputeNextRun_DefaultTZ_NotUTC(t *testing.T) {
 		t.Fatal("computeNextRun returned nil")
 	}
 
-	// With empty TZ (default Asia/Shanghai), 9:00 CST already passed (it's 10:00 CST),
+	// With service default Asia/Shanghai, 9:00 CST already passed (it's 10:00 CST),
-	// so next run should be tomorrow 9:00 CST = today 01:00 UTC + 24h.
+	// so next run should be tomorrow 9:00 CST.
-	// With UTC, 9:00 UTC hasn't happened yet (it's 02:00 UTC), so next run = today 9:00 UTC.
+	// With explicit UTC, 9:00 UTC hasn't happened yet (it's 02:00 UTC).
-	// They must differ.
+	// They must differ, proving SetDefaultTimezone takes effect.
 	if *nextEmpty == *nextUTC {
-		t.Errorf("empty TZ and UTC TZ produced same next run time, default TZ is not working")
+		t.Errorf("service default TZ (Asia/Shanghai) and UTC produced same next run time")
 	}
 }
 
@@ -115,7 +118,7 @@ func TestComputeNextRun_ServiceDefaultTZ(t *testing.T) {
 	now := time.Date(2026, 3, 4, 0, 0, 0, 0, time.UTC)
 	nowMS := now.UnixMilli()
 
-	// Schedule with empty TZ should use service default (US/Eastern), not Asia/Shanghai
+	// Schedule with empty TZ should use service default (US/Eastern), not UTC
 	scheduleEmpty := &CronSchedule{Kind: "cron", Expr: "0 9 * * *", TZ: ""}
 	scheduleCST := &CronSchedule{Kind: "cron", Expr: "0 9 * * *", TZ: "Asia/Shanghai"}