Add optional annual base charge distribution to calculated cost sensors

[Copilot]

Calculated cost sensors currently model only variable consumption cost, which excludes fixed tariff components from hourly/daily/monthly/yearly views. This change adds an optional annual base charge input that is proportionally distributed per configured reset cycle and included directly in calculated sensor values.

• Configuration and schema

  • Added optional annual_calc_base_price config/option field (numeric, default 0).
  • Wired value through config flow and options handling.
  • Added migration/default handling for existing entries to preserve backward compatibility.

• Calculated sensor behavior

  • Extended calculated-cost logic to include:
    • consumption_cost + proportional_base_charge
    • proportional accumulation within the active period (instead of applying full period share at period start).
  • Implemented cycle-based distribution from annual value (hour/day/week/month/bi-monthly/quarterly/half-year/year).
  • Ensured base-charge updates apply forward-only (no retroactive history rewrite).

• State/restore/reset integration

  • Integrated base-charge tracking with reset and restore-state paths so ongoing period progression remains consistent after restart and period boundaries.

• Observability and UX

  • Added attributes exposing configured annual value and current calculated/base-charge context for easier validation in Home Assistant Developer Tools.
  • Added user-facing text/labels for the new option in strings.json and translations (en, de).

calculated_cost = (consumption_kwh * unit_price) + proportional_base_charge_for_current_period

• Fixes Feature proposal: Annual base charge for calculated cost sensors #139