v1.12.0-rc.3 — Native HA form for Grouping Override (pre-release)

Pre-release. Enable "Show beta versions" in HACS.

Fixes the broken-looking Grouping Override section from rc.2.

What changed

• The per-source Grouping Override section is now built from ha-expansion-panel + ha-selector only — the same HA-native primitives the existing Entity Overrides section uses. rc.2's custom <details> + manual <label> markup didn't pick up HA's editor theme correctly, so labels rendered but the input controls beneath them were invisible.
• Each field is now a single ha-selector (number, select, text) with HA's standard label + helper text rendering. Inherit-on-blank semantics still apply — clearing a field drops the key.
• Secondary text on the expansion panel reflects state: "Inherits the card-level Grouping config" when no overrides are set, or "Customized · N override(s)" when there are.
• Clear override button at the bottom of the panel drops the entire override in one click.

Same data model and merge semantics as rc.1 / rc.2 — only the editor markup changed. YAML configs from earlier RCs work as-is.

Test plan (delta from rc.2)

Just verify the editor renders cleanly:

1. Open the card editor → expand any source → expand Grouping Override.
2. Expect: four properly-rendered fields — Window (seconds), Minimum group size, Group by, Group name — each with its label, an actual input control, and helper text below.
3. Type into Window seconds and confirm the value persists when the panel collapses + re-expands.
4. Set Group by to Entity, then re-open the panel → secondary text changes to "Customized · 2 overrides" (or however many fields you set).
5. Click Clear override → all fields blank, secondary text back to "Inherits the card-level Grouping config".

Everything from the rc.1 / rc.2 smoke test still applies to the underlying logic.