v1.3.0 — Water heater, new entities & flows

Feature release: a native water-heater entity plus several new entities and
flows, driven by a dual-AI (Claude + Codex) review for both bugs and features.

Added

• Water heater entity — the tank now appears as a first-class HA
  water_heater: current/target temperature, heat-source operation modes
  (electric / gas / heat pump), and an away (holiday) toggle. Advanced mode.
• Holiday datetime entities — set holiday start/end from a UI picker
  (in addition to the services). Advanced mode.
• Reconfigure flow — update account credentials from the integration's
  Reconfigure button without removing and re-adding the entry.
• Multi-tank picker — the config flow now lists the tanks on your account
  to choose from instead of typing the serial (manual entry still allowed).
• Repair issues — a "tank not found" repair card guides you to reconfigure,
  and clears automatically once the tank is reachable again.
• Device triggers — automate on "hot water low", "heating started/stopped",
  and "holiday started/ended" directly from the Automations UI.
• Configurable alert thresholds — set the low / no hot water percentages in
  the integration options.
• Optional electricity cost sensor — set a price per kWh in options to get a
  running electric-heating cost in your currency.
• quality_scale.yaml checklist tracking Bronze→Platinum rule status.

Changed

• Entity write commands (switches, numbers, select, button, water heater) now
  trigger HA's re-auth flow on an auth failure, consistent with the services.
• Experience-mode selector labels are translatable.
• Diagnostics now include (non-secret) options and coordinator metadata.
• PARALLEL_UPDATES declared on every platform (0 for read, 1 for write).
• Device info now includes the serial number and a configuration URL.

Fixed

• API numeric fields are coerced to finite floats; null/NaN/inf/garbage values
  no longer mislead entities or the energy/cost integrators.
• Options help text corrected to the real 30–300 s poll-interval range.
• electric_power reports a float when idle (was an int 0).
• Services expose an area target in the UI (the backend already resolved it).