v3.1.0-rc7

Release candidate 7 for v3.1.0.

Review-round fixes on top of rc6:

• Atmo: atmoUnavailableLabel / atmoEventLabel now treat the missing-key sentinel from t() as "no localized label", so the fallback chain (Libelle -> noInfoLabel) kicks in instead of the card rendering "card.atmo.unavailable" literally when a translation is absent.
• Card: GP header auto-title reuses the discovery result computed earlier in set hass() instead of running discoverGpSensors() a second time per update. Matches the pattern already used for Atmo.

Nothing else changed since rc6.