Adopt has_entity_name, fix frozen monthly statistics and stale completion readings

[codezorz]

Summary

• Entity naming: Adopt has_entity_name pattern and add configurable device name via options flow, aligning with Home Assistant best practices for entity/device naming
• Fix frozen dashboard statistics: Allow statistics re-injection for the current calendar month so charts update throughout the month instead of freezing at the first injected value
• Fix stale completion readings: Always fetch the authoritative date anchor when a month transitions to COMPLETE_READINGS, instead of relying on cached live-poll readings that can be off by 1 unit; correct the previous month's cache and cascade corrected start values into the current month
• Anonymize docs: Replace real residential unit ID in ENDPOINTS.md example

Fixes #46

Test plan

• All 231 existing tests pass after changes
• New test: current month always bypasses the statistics injection guard
• New tests: anchor is always fetched at completion (5 scenarios: always fetched, cache corrected when anchor differs, current month starts cascade, no-op when anchor matches, fallback on API failure)
• Verify dashboard chart updates throughout the current month after HA restart
• Verify that at month boundary, the previous month's total_usage matches the API's totalEnergyUsage value