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/VERSIONS.json

@@ -0,0 +1,9 @@
+{
+  "project": "zpark-ems",
+  "project_version": "1.3.0",
+  "customer": "Z-Park 中关村医疗器械园",
+  "core_version": "1.1.0",
+  "frontend_template_version": "1.1.0",
+  "last_updated": "2026-04-06",
+  "notes": "Z-Park branding, ps_id fix, feature flags, buyoff CONDITIONAL PASS"
+}