fix(transport): auto-derive v1r login password from full gateway password

[Bre77]

Intent

• Fixes a silent 401 usability trap in local v1r login: the gateway's /api/login/Basic only accepts the last 5 characters of the gateway/WiFi password, not the full value.
  • Discovered live against real Powerwall 3 hardware: the full saved gateway password 401s, the last-5-chars truncation succeeds first try.
  • This is a known, established Powerwall local-login convention - jasonacox/pypowerwall's v1r path auto-derives it the same way, rather than documenting it as a caller burden.
  • aiopowerwall previously took gateway_password and passed it straight through unverbatim, so every caller had to already know the last-5 trick out-of-band or get a bare 401 with no hint why.
• Fix: V1rTransport now derives the 5-char customer password internally (_customer_password, truncates to the last 5 characters). PowerwallClient(gateway_password=...) keeps taking the full gateway/WiFi password - the value callers already have and the value the constructor docstring and README document.
  • No public API change - just correct behavior for the documented parameter.
  • README and constructor docstring now say explicitly that the full password is expected and the truncation is automatic.
• Tests: unit-tests the truncation helper and asserts V1rTransport.login() sends the derived 5-char value over the wire, given a full password input. No live-hardware test added (per the brief, the live proof already exists).