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
- Add ErrFutureDay sentinel error
- CreateInterval: rejects if startDayKey > todayKey (400)
- Update: rejects if new start_time moves entry to a future day (400)
- Handler maps ErrFutureDay → 400 Bad Request for both endpoints
- Add TestCreateIntervalRejectsFutureDay
- Add TestUpdateRejectsMoveToFutureDay
- UI already gates this via dayCapabilities (canAddInterval=false,
canEditEntries=false for future days), but server now enforces it too
