Fix plantmode write no-op on Elco Aerotop device classified as GALEVO instead of bsb

[albertorosso]

(partially generated with claude AI)

Some devices classified by the cloud as SystemType.GALEVO (observed on an Elco Aerotop heat pump with a BSB controller) accept plant mode writes via the REST v2 endpoint (POST /api/v2/remote/dataItems/{gw}/set) and receive {"success": true}, but the command has no real effect on the physical device.

The official web client (remocon-net.remotethermo.com), for this same gateway, never uses the REST v2 write path at all — it uses a legacy ASP.NET MVC-style endpoint:

POST /R2/PlantHome/GetData/{gatewayId}?umsys={umsys}   (read)
POST /R2/PlantHome/SetData/{gatewayId}?umsys={umsys}   (write)

```confirmed via HAR capture of a successful "set to OFF" action (response changedItems shows PlantMode → 5.0).
This may be related to #299 and #424 (on the other repository) which describe similar symptoms on Elco/Ariston heat pump devices.

**Root cause**
The device's cloud-reported system_type is GALEVO, so AristonGalevoDevice (not AristonBsbDevice) is instantiated, and its async_set_plant_mode uses the modern REST v2 write path — which the cloud silently accepts but does not propagate to this device's physical unit. Reads work fine on REST v2; only writes are affected.

**Fix**

- Added legacy_r2_client.py: an isolated client for the legacy /R2/ read/write path (separate session/auth from the main AristonAPI, since auth mechanisms differ - session cookies vs bearer token).

- AristonGalevoDevice.async_set_plant_mode now verifies the write by re-reading state (async_update_state) shortly after the REST v2 write; if the value didn't actually change, it falls back automatically to the legacy client.

- Added ariston_api.py public username/password properties so the legacy client can reuse the already-configured credentials without extra user configuration.

- Bundled default_viewmodel_template.json: the site does not expose a clean read endpoint for the full viewModel object required by the legacy write (it's built client-side in JS from the initial page render). This is a pragmatic starting point captured from a real working request, with device-specific fields (gatewayId) overwritten dynamically at call time. 

- Known limitation: this template may not generalize cleanly to all GALEVO/BSB device variants - a more general future fix could parse the live viewModel from the initial /R2/Plant/Index page HTML instead of relying on a static bundled capture.