feat(covers): close the covers on the sun's elevation, and name those…

[Gamso]

… left up

Two changes to the evening close, kept in one commit because they land in the same files and the same screens.

The closing time now comes from the sun's elevation. "Sunset + N minutes" is a delay you have to guess at; degrees above the horizon are a light level, the value sun.sun already publishes. Covers close when the descending sun reaches the configured elevation. Degrees say nothing about clock time, so the form turns the setting into tonight's time from the instance's own location — "At -2°, that is 21:41 tonight."

The minute offset is gone rather than kept alongside: two settings for one decision, with only one of them live, is a trap. The v3 -> v4 migration converts each entry's stored offset into the elevation it was landing on, averaged over the equinoxes and solstices and rounded to the half degree the form offers, so covers keep moving at the time they moved before. Measured with astral at 43.5N: sunset + 10 min is -2.5, sunset - 10 min is +1.5. The range offered (+-25) holds every value the old +-120 minutes could express, and an entry that never touched the offset is converted from the default it was silently running on.

Default is -2°: that is where the old default of sunset + 10 min landed, at every season, so a fresh install behaves like the old one.

The form refuses an elevation the sun never reaches at that location, checked against both solstices, instead of leaving the runtime to discover it at nightfall months later. What that rejects depends entirely on latitude: nothing at 43N, where the sun sweeps the whole range every day; +10 at 60N, where the midwinter sun stops at 6.6; everything above the polar circle in June. A rejected submission redisplays what was typed. The runtime keeps a fallback to plain sunset for what the form cannot check — a home that moved, an entry restored elsewhere — and the Cover Close Time sensor reports which of the two produced tonight's time.

binary_sensor.covers_left_open raises the skipped-cover warning where it can be acted on. A cover whose window sensor reads open is left up and, until now, only mentioned in the log, which nobody reads. The entity is a device_class problem, on as soon as one cover was skipped, listing the covers and the sensor that blocked each of them so a notification can name them. It clears when the next day's schedule is computed, not when the window closes: closing the window does not bring the cover down, so the warning has to outlive its cause. The list is persisted, so a reboot the same evening does not silence it.

Supporting changes: _covers_behind_an_open_window returns {cover: sensor} rather than a set, since the entity has to name the sensor and the callers only needed membership; _async_run_cover_checks pushes state to the entities, because the open/close timers fire between polls and the warning would otherwise show the previous value for up to five minutes; the astral lookups live at module level in cover_manager so the config flow and the migration can use them without a CoverManager, and go through get_astral_observer, since get_astral_location is deprecated for removal in HA 2027.7.

59 new tests (338 -> 397).