Update and enhancements!

Release Summary v0.1.6.5 — v0.1.8.5

Big updates and rewrites!

Improved setup and options validation:
Better API token validation.
Clear handling of unauthorized serials.

Added robust API error handling:
Proper handling of Solax rate-limit responses.
Better mapping of unauthorized/invalid-serial cases.
Clearer logging and user-facing diagnostics.

Improved reload behavior and reliability:
Keeps previous valid data during temporary API/rate-limit issues.
On options save, only newly added/changed inverters are refreshed first (when token is unchanged).
Avoids unnecessary full data loss on config changes.

Added user-facing safety feedback:
Options-flow popup for rate-limit events.
Options-flow popup for invalid serial/access events.
Persistent notifications for both rate limit and invalid serial/access.

Added notification control:
New API Rate Limit Notifications switch in System Totals.
Notifications are shown/hidden based on this toggle.

Improved diagnostics model:
Added per-inverter API Access Status diagnostic sensor.
System Health and API Rate Limit Status moved to diagnostics in System Totals.
Last Poll Attempt and Next Scheduled Poll kept as diagnostics and disabled by default.

Sensor/model cleanup improvements:
Dynamic entity creation only for non-null API fields.
Removed inverter serials now clean up stale entities/devices from registry.
Better handling of unavailable states for invalid serials.

Architecture and HA best-practice improvements:
Single config-entry behavior enforced.
Better translation handling and friendly-name consistency.
Preflight setup optimization to reduce unnecessary extra API round-trips.
Documentation refresh:

README updated with current behavior, diagnostics, notifications, rate-limit handling, and supported inverter model names.