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/handler/settings_handler.go

@@ -1,8 +1,10 @@
 package handler
 
 import (
+	"database/sql"
 	"errors"
 	"net/http"
+	"strconv"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/wotra/wotra/internal/service"
@@ -21,6 +23,8 @@ func (h *SettingsHandler) Routes(r chi.Router) {
 	r.Get("/settings", h.Current)
 	r.Put("/settings", h.Upsert)
 	r.Get("/settings/history", h.History)
+	r.Put("/settings/history/{id}", h.UpdateHistoryRow)
+	r.Delete("/settings/history/{id}", h.DeleteHistoryRow)
 }
 
 // Current GET /api/settings
@@ -76,3 +80,62 @@ func (h *SettingsHandler) History(w http.ResponseWriter, r *http.Request) {
 	}
 	writeJSON(w, http.StatusOK, history)
 }
+
+// UpdateHistoryRow PUT /api/settings/history/{id}
+func (h *SettingsHandler) UpdateHistoryRow(w http.ResponseWriter, r *http.Request) {
+	id, err := strconv.ParseInt(chi.URLParam(r, "id"), 10, 64)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid id")
+		return
+	}
+	var body struct {
+		EffectiveFrom string  `json:"effective_from"`
+		HoursPerWeek  float64 `json:"hours_per_week"`
+		WorkdaysMask  int     `json:"workdays_mask"`
+		Timezone      string  `json:"timezone"`
+	}
+	if err := decodeJSON(r, &body); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid JSON")
+		return
+	}
+	set, err := h.svc.UpdateSettings(r.Context(), id, service.UpdateSettingsInput{
+		EffectiveFrom: body.EffectiveFrom,
+		HoursPerWeek:  body.HoursPerWeek,
+		WorkdaysMask:  body.WorkdaysMask,
+		Timezone:      body.Timezone,
+	})
+	if err != nil {
+		switch {
+		case errors.Is(err, service.ErrSettingsNotFound), errors.Is(err, sql.ErrNoRows):
+			writeError(w, http.StatusNotFound, err.Error())
+		case errors.Is(err, service.ErrInvalidHours), errors.Is(err, service.ErrInvalidWorkdaysMask):
+			writeError(w, http.StatusUnprocessableEntity, err.Error())
+		default:
+			writeError(w, http.StatusInternalServerError, err.Error())
+		}
+		return
+	}
+	writeJSON(w, http.StatusOK, set)
+}
+
+// DeleteHistoryRow DELETE /api/settings/history/{id}
+func (h *SettingsHandler) DeleteHistoryRow(w http.ResponseWriter, r *http.Request) {
+	id, err := strconv.ParseInt(chi.URLParam(r, "id"), 10, 64)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid id")
+		return
+	}
+	if err := h.svc.DeleteSettings(r.Context(), id); err != nil {
+		switch {
+		case errors.Is(err, service.ErrSettingsNotFound):
+			writeError(w, http.StatusNotFound, err.Error())
+		case errors.Is(err, service.ErrLastSettingsRow):
+			writeError(w, http.StatusConflict, err.Error())
+		default:
+			writeError(w, http.StatusInternalServerError, err.Error())
+		}
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+