UX-44: contract label says every 15 minutes, not hourly

The settings page contract choice read "Dynamic, hourly" / "Dynamisch,
per uur". Belgian day-ahead prices are per 15 minutes since 2025-10-01.
Now "Dynamic, every 15 minutes" / "Dynamisch, per kwartier". One DOCS
line that called dynamic pricing "hourly pricing" now says dynamic.
test_ui_text.py part 7 fails if either label says hourly again.

Co-Authored-By: Claude <noreply@anthropic.com>

goodwe_controller/app/ui_text/en.json

@@ -29,7 +29,7 @@
     "periods": {"name": "Time windows and prices", "description": "One price per kWh for each time window, all in, as on your bill: divide c€/kWh by 100, so 34.512 c€/kWh is 0.34512."}
   },
   "choices": {
-    "contract": {"none": "No prices", "fixed": "Fixed price", "daynight": "Day and night", "dynamic": "Dynamic, hourly", "custom": "Other time windows"},
+    "contract": {"none": "No prices", "fixed": "Fixed price", "daynight": "Day and night", "dynamic": "Dynamic, every 15 minutes", "custom": "Other time windows"},
     "fallback": {"none": "None", "fixed": "One price", "daynight": "Day and night", "custom": "Other time windows"},
     "region": {"flanders": "Flanders, Fluvius Antwerpen", "own": "I enter my own figure"},
     "meter_source": {"off": "One grid power entity", "ha_dsmr": "Import and export entities", "mqtt_p1": "Meter over MQTT", "ha_signed": "One net power entity", "homewizard_local": "HomeWizard meter on the network"},