SAFETY-79: peak guard keeps a margin below the limit

New option peak_margin_w (default 50 W, 0-500 W). The peak guard, the
forecast checks and the maintenance charge sizing aim at
peak_cap_w - peak_margin_w; the month peak, the bill, headroom, warnings
and every notice keep the real cap. Owner decision 2026-09-28.

peak_margin_w 0 is byte-identical to ed739d4 (tools/safety79_identity.py,
test_peak_margin.py part 1). validate.py rule 12 refuses a value outside
0-500, not whole, or not below the peak limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UBtqPrG1mDLXzTEBnhEkkV

goodwe_controller/app/ui_text/en.json

@@ -13,7 +13,7 @@
     "inverters.modes": {"name": "Battery modes"},
     "tariff.selling": {"name": "Charging and selling on price"},
     "tariff.feed": {"name": "Price feed details"},
-    "safety.peak": {"name": "Peak limit"},
+    "safety.peak": {"name": "Peak limit", "description": "The add-on aims a little below your limit, to be safe."},
     "safety.start": {"name": "Starting and stopping"},
     "safety.stops": {"name": "Safety stops", "description": "Measured on real hardware, so leave them unchanged."},
     "maintenance.support": {"name": "Support"}