feat: Dashboard-Metriken nach Mailpiler-Vorbild (Uptime, Aktivität, Prognose)

- storage_stats.go (neu): MailActivityStats (60min/24h/7d/30d), StorageEstimateStats
- dashboard_handlers.go: Uptime (/proc/uptime), activity + estimate in System-Stats-Response
- DashboardTab: Uptime in API-Kachel, neue Kacheln "Mail-Eingang" + "Speicherprognose"
- Warnung (Badge "Knapp!") wenn Partition in <90 Tagen voll

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/dashboard_handlers.go

@@ -154,6 +154,17 @@ func (s *Server) handleSystemStats(w http.ResponseWriter, r *http.Request) {
 		disks = []diskStat{}
 	}
 
+	// Uptime: /proc/uptime
+	uptimeResp := map[string]interface{}{"seconds": 0.0}
+	if data, err := os.ReadFile("/proc/uptime"); err == nil {
+		parts := strings.Fields(string(data))
+		if len(parts) >= 1 {
+			if secs, err := strconv.ParseFloat(parts[0], 64); err == nil {
+				uptimeResp["seconds"] = secs
+			}
+		}
+	}
+
 	// Archive: first & last mail
 	archiveResp := map[string]interface{}{"first_mail": nil, "last_mail": nil}
 	first, last, err := s.store.FirstAndLastMail()
@@ -166,11 +177,25 @@ func (s *Server) handleSystemStats(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// Mail activity (60min / 24h / 7d / 30d)
+	activity, _ := s.store.MailActivityStats(r.Context())
+
+	// Storage estimate — use free bytes of the first non-root disk (or root)
+	var freeDiskBytes uint64
+	for _, d := range disks {
+		freeDiskBytes = d.FreeBytes
+		break
+	}
+	estimate, _ := s.store.StorageEstimateStats(r.Context(), freeDiskBytes)
+
 	writeJSON(w, http.StatusOK, map[string]interface{}{
-		"cpu":     cpuResp,
-		"ram":     ramResp,
-		"disks":   disks,
-		"archive": archiveResp,
+		"cpu":      cpuResp,
+		"ram":      ramResp,
+		"disks":    disks,
+		"uptime":   uptimeResp,
+		"archive":  archiveResp,
+		"activity": activity,
+		"estimate": estimate,
 	})
 }