feat: Improve config flow validation, error handling, and reconfigure UX

[davidnoyes]

Summary

• Host validation: strips whitespace and rejects URLs containing :// or / before attempting a connection, giving immediate feedback rather than a confusing timeout
• Granular error messages: maps connection failures to specific error keys (invalid_host / cannot_connect / invalid_inverter) with user-friendly translated messages
• Always close on failure: moves the serial number read inside the try block so client.close() is guaranteed to run even when detection fails
• Unique ID: sets the inverter serial number as the config entry unique ID on first setup, allowing HA to detect and prevent duplicate entries
• Reconfigure step: adds async_step_reconfigure so users can update the inverter host in-place (e.g. after a DHCP change) without removing and re-adding the integration
• Stale translation cleanup: removes the orphaned num_batteries translation key left behind when the field was removed from the config flow schema in Write settings using services #4

Test plan

• Enter an invalid host (e.g. http://192.168.1.1) — should show invalid_host error immediately
• Enter a hostname that doesn't resolve — should show cannot_connect
• Enter a reachable host that isn't a GivEnergy inverter — should show invalid_inverter
• Add the integration twice with the same inverter — should abort with already_configured
• Use the Reconfigure option on an existing entry to change the host