feat: startup health check during splash screen (v1.7.20)

- Add ?action=health_check PHP endpoint (early-exit, before rate-limiter)
  Checks: PHP version, required extensions, data/ writability, SQLite DB
  connection + table integrity, .env file, Gemini AI key, Bring! token
- Display animated checklist in splash screen with per-item icons
  (ok/warn/error); critical failures block app launch with clear error
  message and Retry button; optional warnings shown but don't block
- New JS: _runStartupCheck(), _startupRetry(); called first in _initApp()
- New HTML elements in #app-preloader: #preloader-checks, #preloader-error-msg,
  #preloader-retry-btn (hidden until startup check completes)
- New CSS: .preloader-checks, .preloader-check-row, .preloader-error-msg,
  .preloader-retry-btn with state colors (ok=green, warn=amber, error=red)
- Translations: startup.* keys (10 per language) in IT, EN, DE, FR, ES
- Asset version bump: v=20260520a

manifest.json

@@ -2,7 +2,7 @@
     "name": "EverShelf",
     "short_name": "EverShelf",
     "description": "Gestione completa della dispensa di casa con scansione barcode",
-    "version": "1.7.19",
+    "version": "1.7.20",
     "start_url": "/evershelf/",
     "display": "standalone",
     "background_color": "#f0f4e8",