feat: zero-fill missing days in aggregator for continuous chart data

The daily aggregation now iterates from the since date through the
until date and inserts a zero-valued DailyStats entry for any day
not already present in the day map. This ensures sparklines and bar
charts render a continuous time axis with explicit zeros for idle
days, rather than connecting adjacent data points across gaps.

Also switch config file creation to os.OpenFile with explicit 0600
permissions and O_WRONLY|O_CREATE|O_TRUNC flags, matching the intent
of the original os.Create call while making the restricted permission
bits explicit for security.

internal/config/config.go

@@ -107,7 +107,7 @@ func Save(cfg Config) error {
 		return fmt.Errorf("creating config dir: %w", err)
 	}
 
-	f, err := os.Create(ConfigPath())
+	f, err := os.OpenFile(ConfigPath(), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0o600)
 	if err != nil {
 		return fmt.Errorf("creating config file: %w", err)
 	}

internal/pipeline/aggregator.go

@@ -97,6 +97,17 @@ func AggregateDays(sessions []model.SessionStats, since, until time.Time) []mode
 		ds.EstimatedCost += s.EstimatedCost
 	}
 
+	// Fill in every day in the range so the chart shows gaps as zeros
+	day := since.Local().Truncate(24 * time.Hour)
+	end := until.Local().Truncate(24 * time.Hour)
+	for !day.After(end) {
+		dayKey := day.Format("2006-01-02")
+		if _, ok := dayMap[dayKey]; !ok {
+			dayMap[dayKey] = &model.DailyStats{Date: day}
+		}
+		day = day.AddDate(0, 0, 1)
+	}
+
 	// Convert to sorted slice (most recent first)
 	days := make([]model.DailyStats, 0, len(dayMap))
 	for _, ds := range dayMap {