OPT-34: the night charge keeps its level when the kept forecast ends

With the forecast sensors silent from 17:05 the kept evening figure ended at
midnight + 3 h, in the middle of the night grid charge, and Leftover solar's
planned level went with it (rig: A 49.8 % against 79.9 %).

- A night grid charge that has started keeps its planned level when the forecast
  goes (Controller._latch_charge, store key night_charge_latch), and the narrow
  price gap with it (weather_held). A fresh figure still moves the level.
- The evening's tomorrow figure is today's until the bell starts (06:00 local),
  up to 15 h old but only if it was usable at midnight (solarfc.Hold.stand_in).
- Screen: "No solar forecast now. The charge under way goes on to its planned
  level." (EN/NL) while a latched battery is below its level.
- test_opt34.py (whole nights through the real control cycle, mutants), test_opt27
  and test_modes_p4 adjusted to the new window and the kept level.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bdqr7PiaHcBnzPituu2yTC

goodwe_controller/app/ui_text/en.json

@@ -360,7 +360,7 @@
       "tops_up": "It charges from the grid to {pct}. That covers {kwh} of expensive hours the sun will not.",
       "no_forecast": "No solar forecast, so it charges from solar only."
     },
-    "solar": {"line": "Solar still to come today: {kwh}, we count on {count}. Updated {at}.", "uncovered": "The expensive hours need more than Leftover solar can hold. The rest, {kwh}, comes from another battery or the grid.", "kept": "Solar forecast read {at}, kept for tonight.", "no_forecast": "No solar forecast now, so Leftover solar charges from solar only.", "learning": "The add-on is learning your house's use. The plan starts after one full day."},
+    "solar": {"line": "Solar still to come today: {kwh}, we count on {count}. Updated {at}.", "uncovered": "The expensive hours need more than Leftover solar can hold. The rest, {kwh}, comes from another battery or the grid.", "kept": "Solar forecast read {at}, kept for tonight.", "no_forecast": "No solar forecast now, so Leftover solar charges from solar only.", "latched": "No solar forecast now. The charge under way goes on to its planned level.", "learning": "The add-on is learning your house's use. The plan starts after one full day."},
     "today": {
       "not_planned": "Not planned yet.",
       "no_runs": "No cheap or expensive hours left today.",