feat: add admin screens, permissions and navigation

Seven admin controllers in a "Studia BM" navigation group, guarded by
jz.bm.access_courses, jz.bm.access_schedule and jz.bm.access_materials,
with Polish and English labels. Date and time fields are checked by the
models before a save and answered as 422 on the field.

controllers/admin_test.go

@@ -0,0 +1,35 @@
+package controllers
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/cabana"
+	"git.golem15.com/jakub/sm-bm-plugin/models"
+)
+
+func TestTemporalHooksAnswerOnTheField(t *testing.T) {
+	var hooks temporalHooks
+	bad := &models.ClassSession{StartsAt: "2026-10-03 08:00", EndsAt: "wtorek"}
+	for name, fn := range map[string]func(context.Context, any) error{
+		"create": hooks.FormBeforeCreate,
+		"update": hooks.FormBeforeUpdate,
+	} {
+		err := fn(context.Background(), bad)
+		var ve *cabana.ValidationError
+		if !errors.As(err, &ve) {
+			t.Fatalf("%s: err = %v, want a cabana.ValidationError", name, err)
+		}
+		if _, ok := ve.Details["ends_at"]; !ok {
+			t.Errorf("%s: details %v name no ends_at", name, ve.Details)
+		}
+	}
+	good := &models.ClassSession{StartsAt: "2026-10-03 08:00", EndsAt: "2026-10-03 09:30"}
+	if err := hooks.FormBeforeCreate(context.Background(), good); err != nil {
+		t.Errorf("valid session: %v", err)
+	}
+	if err := hooks.FormBeforeUpdate(context.Background(), &models.Tag{}); err != nil {
+		t.Errorf("model without dates: %v", err)
+	}
+}