feat(tasks): due dates with optional time; alarms at the right moment

Due dates can carry a time (YYYY-MM-DDTHH:MM). Alarms fire at that time,
or 09:00 for all-day tasks, minus the user's lead time, in TZ (default
Europe/Berlin, tzdata embedded). A new due date or reopening a task
re-arms the alarm. Recurrence keeps the time; Markdown export/import
carry it. The UI shows relative labels ("Heute, 14:30"), marks overdue
tasks to the minute and groups by local day instead of the UTC date.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/server/main.go

@@ -11,6 +11,7 @@ import (
 	"os/signal"
 	"syscall"
 	"time"
+	_ "time/tzdata" // time zones without system tzdata (scratch/alpine images)
 
 	"github.com/sandrodach/mindsift/internal/api"
 	"github.com/sandrodach/mindsift/internal/config"
@@ -22,6 +23,12 @@ import (
 var embeddedDist embed.FS
 
 func main() {
+	// Due dates and alarms use the server's local time; without TZ that is Europe/Berlin
+	if os.Getenv("TZ") == "" {
+		if loc, err := time.LoadLocation("Europe/Berlin"); err == nil {
+			time.Local = loc
+		}
+	}
 	cfg := config.Load()
 
 	level := cfg.SlogLevel()