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

assets/css/style.css

@@ -116,6 +116,55 @@ body {
     letter-spacing: 0.5px;
     margin-top: -8px;
 }
+/* ── Startup health check list ─────────────────────────────────────── */
+.preloader-checks {
+    display: flex;
+    flex-direction: column;
+    gap: 6px;
+    width: 240px;
+    max-width: 90vw;
+    animation: zwFadeIn 0.25s ease;
+}
+.preloader-check-row {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    font-size: 0.82rem;
+    color: rgba(255,255,255,0.80);
+    background: rgba(255,255,255,0.07);
+    border-radius: 8px;
+    padding: 5px 10px;
+    transition: background 0.2s;
+}
+.preloader-check-row[data-state="ok"]    { background: rgba(74,222,128,0.10); color: rgba(255,255,255,0.92); }
+.preloader-check-row[data-state="warn"]  { background: rgba(251,191,36,0.12); color: rgba(255,255,255,0.92); }
+.preloader-check-row[data-state="error"] { background: rgba(239,68,68,0.15);  color: #fca5a5; }
+.pck-icon  { font-size: 1rem; line-height: 1; flex-shrink: 0; }
+.pck-label { white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
+.preloader-error-msg {
+    color: #fca5a5;
+    background: rgba(239,68,68,0.18);
+    border: 1px solid rgba(239,68,68,0.4);
+    border-radius: 10px;
+    padding: 10px 16px;
+    font-size: 0.88rem;
+    text-align: center;
+    max-width: 280px;
+    line-height: 1.4;
+    animation: zwFadeIn 0.3s ease;
+}
+.preloader-retry-btn {
+    background: #ef4444;
+    color: #fff;
+    border: none;
+    border-radius: 8px;
+    padding: 9px 22px;
+    font-size: 0.9rem;
+    font-weight: 600;
+    cursor: pointer;
+    animation: zwFadeIn 0.3s ease;
+}
+.preloader-retry-btn:active { opacity: 0.8; }
 .header-logo-icon {
     height: 28px;
     width: auto;