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/hooks.go

@@ -0,0 +1,55 @@
+package controllers
+
+import (
+	"context"
+
+	"git.golem15.com/golem15/summercms/modules/cabana"
+	"git.golem15.com/jakub/sm-bm-plugin/models"
+)
+
+// Permission codes of the plugin's admin screens.
+const (
+	PermCourses   = "jz.bm.access_courses"
+	PermSchedule  = "jz.bm.access_schedule"
+	PermMaterials = "jz.bm.access_materials"
+)
+
+// temporalHooks checks a model's Date and DateTime columns before every admin
+// save and answers a bad value as a 422 on its field. lagoon.Validate has no
+// date rules, so the model checks them itself (models.TemporalValidator).
+type temporalHooks struct{}
+
+func (temporalHooks) FormBeforeCreate(_ context.Context, model any) error {
+	return checkTemporal(model)
+}
+
+func (temporalHooks) FormBeforeUpdate(_ context.Context, model any) error {
+	return checkTemporal(model)
+}
+
+func checkTemporal(model any) error {
+	v, ok := model.(models.TemporalValidator)
+	if !ok {
+		return nil
+	}
+	msgs := v.TemporalErrors()
+	if len(msgs) == 0 {
+		return nil
+	}
+	details := make(map[string]any, len(msgs))
+	for field, m := range msgs {
+		details[field] = m
+	}
+	return &cabana.ValidationError{Details: details}
+}
+
+// courseRelation is the belongsTo picker of a course_id column.
+func courseRelation(foreignKey string) cabana.FieldRelationContract {
+	return cabana.FieldRelationContract{
+		Field:       "course",
+		Kind:        "belongsTo",
+		NewRelated:  func() any { return &models.Course{} },
+		ForeignKey:  foreignKey,
+		LabelColumn: "name",
+	}
+}