Overview device rename, current-power naming, and roof icons

[patricknitsch]

Summary

• The device that groups all summed/total-across-roofs sensors (total power, total energy, total clear-sky power, temperature, weather code) was named after the config entry title (usually just "pvnode") - the same name as the integration itself, which didn't convey that this specific device holds the combined/site-wide values.
  • Switch PvnodeTotalEntity's DeviceInfo from a fixed name=entry.title to translation_key="overview", resolved via a new device.overview.name translation entry: "Summary" in English, "Gesamt" in German.
• Rename the "power"/"total_power" entities from "Power forecast"/"Total power forecast" to "Current power forecast"/"Total current power forecast" (German: "Leistungsprognose aktuell"/"Gesamt-Leistungsprognose aktuell"), to make clear these represent the current time slot's value, not the whole forecast.
• Add icons.json giving every per-roof-surface sensor (power, energy per day, clear-sky power) the mdi:home-roof icon, so they're visually recognizable as belonging to a physical roof surface. The summary/overview device's sensors keep their existing device-class-driven icons (unchanged).
• Updated the README's "Entities" and "Use Cases" sections to match.

Compatibility note

These only change displayed names/icons - existing installations keep their current entity_ids (HA doesn't rename already-registered entities when the underlying device/entity name or translation changes), so this is not a breaking change for anyone upgrading. Only brand-new setups (or the test suite, which spins up fresh entities every time) get entity_ids derived from the new names, e.g. sensor.summary_total_current_power_forecast / sensor.dachflache_1_current_power_forecast instead of sensor.pvnode_total_power_forecast / sensor.dachflache_1_power_forecast.

Test plan

• pytest tests/ -v — all 22 tests pass (entity_id assertions updated for the new names)
• ruff check / ruff format --check — clean
• Verified strings.json / translations/en.json / translations/de.json / icons.json all parse as valid JSON
• Note: icons.json-driven default icons are resolved by the HA frontend, not exposed as a backend state attribute, so this can't be asserted directly in the pytest harness - verified by reading HA core's helpers/icon.py / helpers/entity.py to confirm the resolution mechanism and file-discovery convention (no manifest change needed).