feat: solar KPIs, version display, feature flags (v1.4.0)

New dashboard KPI cards:
- Performance Ratio (PR) with color thresholds
- Equivalent Utilization Hours
- Daily Revenue (¥)
- Self-Consumption Rate

Version display for field engineers:
- Login page footer: "v1.4.0 | Core: v1.4.0"
- Sidebar footer: version when expanded
- System Settings: full version breakdown

Backend (core sync):
- GET /api/v1/version (no auth) — reads VERSIONS.json
- GET /api/v1/kpi/solar — PR, revenue, equiv hours calculations
- Dashboard energy_today fallback from raw energy_data
- PV device filter includes sungrow_inverter type

Feature flags:
- Sidebar hides disabled features (charging, bigscreen_3d)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/backend/app/api/router.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.api.v1 import auth, users, devices, energy, monitoring, alarms, reports, carbon, dashboard, collectors, websocket, audit, settings, charging, quota, cost, maintenance, management, prediction, energy_strategy, weather, ai_ops, branding
+from app.api.v1 import auth, users, devices, energy, monitoring, alarms, reports, carbon, dashboard, collectors, websocket, audit, settings, charging, quota, cost, maintenance, management, prediction, energy_strategy, weather, ai_ops, branding, version, kpi
 
 api_router = APIRouter(prefix="/api/v1")
 
@@ -26,3 +26,5 @@ api_router.include_router(energy_strategy.router)
 api_router.include_router(weather.router)
 api_router.include_router(ai_ops.router)
 api_router.include_router(branding.router)
+api_router.include_router(version.router)
+api_router.include_router(kpi.router)