E3DC: address PR review feedback, add config check

Changes based on PR #25703 review:
- Add context.Context parameter to constructor (Go conventions)
- Improve port parsing error handling
- Fix code duplication: powers() is now private, CurrentPower() uses it
- Add getExternDataAlg() and getSessionData() helpers to reduce duplication
- Add checkConfiguration() to verify wallbox settings at startup:
  - Sun mode: automatically disabled (conflicts with evcc control)
  - Auto phase switching: warning only (user may want to keep it)
- Add header documentation with configuration requirements
- Rename port_ to portStr for clarity

Phase switching (1p/3p) tested successfully with real hardware.
The wallbox handles ramping internally - evcc only needs to send the command.

Author: D3c1mu2