feat: edit and delete settings history rows

Backend:
- SettingsStore: Add GetByID, Update, Delete, Count methods
- SettingsService: Add UpdateSettings (validates same rules as Upsert),
  DeleteSettings (guards against deleting the last row → 409)
- New sentinels: ErrSettingsNotFound, ErrLastSettingsRow
- Handler: PUT /api/settings/history/{id} → 200 updated row
           DELETE /api/settings/history/{id} → 204 / 404 / 409

Frontend:
- API client: settings.update(id, body) and settings.delete(id)
- Settings page: history table gains edit (pencil) and delete (×) buttons
- Inline edit form expands in place within the table row
- Delete button disabled and hint shown when only one row remains
- maskLabel() helper shows workday names instead of raw bitmask
- After save/delete: full reload to reflect changes in 'current' section

internal/service/settings_service.go

@@ -2,6 +2,7 @@ package service
 
 import (
 	"context"
+	"database/sql"
 	"errors"
 	"fmt"
 	"time"
@@ -14,6 +15,8 @@ var (
 	ErrNoSettings          = errors.New("no settings found")
 	ErrInvalidWorkdaysMask = errors.New("workdays_mask must be between 1 and 127")
 	ErrInvalidHours        = errors.New("hours_per_week must be > 0")
+	ErrSettingsNotFound    = errors.New("settings row not found")
+	ErrLastSettingsRow     = errors.New("cannot delete the only settings row")
 )
 
 // SettingsService manages settings with effective-from history.
@@ -87,3 +90,67 @@ func (s *SettingsService) Upsert(ctx context.Context, input UpsertSettingsInput)
 	}
 	return set, nil
 }
+
+// UpdateSettingsInput is the payload for editing an existing settings row.
+type UpdateSettingsInput struct {
+	EffectiveFrom string
+	HoursPerWeek  float64
+	WorkdaysMask  int
+	Timezone      string
+}
+
+// UpdateSettings edits an existing settings row in-place.
+func (s *SettingsService) UpdateSettings(ctx context.Context, id int64, input UpdateSettingsInput) (*domain.Settings, error) {
+	if input.HoursPerWeek <= 0 {
+		return nil, ErrInvalidHours
+	}
+	if input.WorkdaysMask < 1 || input.WorkdaysMask > 127 {
+		return nil, ErrInvalidWorkdaysMask
+	}
+	if input.Timezone == "" {
+		input.Timezone = "UTC"
+	}
+	if _, err := time.LoadLocation(input.Timezone); err != nil {
+		return nil, fmt.Errorf("invalid timezone: %w", err)
+	}
+	if _, err := time.Parse("2006-01-02", input.EffectiveFrom); err != nil {
+		return nil, fmt.Errorf("invalid effective_from: %w", err)
+	}
+
+	set, err := s.store.GetByID(ctx, id)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, ErrSettingsNotFound
+		}
+		return nil, err
+	}
+
+	set.EffectiveFrom = input.EffectiveFrom
+	set.HoursPerWeek = input.HoursPerWeek
+	set.WorkdaysMask = input.WorkdaysMask
+	set.Timezone = input.Timezone
+
+	if err := s.store.Update(ctx, set); err != nil {
+		return nil, err
+	}
+	return set, nil
+}
+
+// DeleteSettings removes a settings row. Refuses if it is the only row.
+func (s *SettingsService) DeleteSettings(ctx context.Context, id int64) error {
+	count, err := s.store.Count(ctx)
+	if err != nil {
+		return err
+	}
+	if count <= 1 {
+		return ErrLastSettingsRow
+	}
+	// Confirm row exists before deleting.
+	if _, err := s.store.GetByID(ctx, id); err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return ErrSettingsNotFound
+		}
+		return err
+	}
+	return s.store.Delete(ctx, id)
+}