fix: Error code 380 + dynamic fallback for unknown codes (#16)

- Expand ERROR_CODES from 15 to 294 entries based on official
  Hargassner Nano-PK manual (shb-nano-pk-6-32-de.pdf)
- Fix error 380: "Wartung fällig" (not "Sicherheitstemperaturbegrenzer")
- Correct existing descriptions to match manual wording
- Add dynamic fallback in HargassnerErrorSensor for unknown error
  codes, preventing HA ENUM validation from showing "Unknown"

Author: karlspace