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

index.html

@@ -11,7 +11,7 @@
     <title>EverShelf</title>
     <link rel="manifest" href="manifest.json">
     <link rel="icon" type="image/png" href="assets/img/logo/logo_icon.png">
-    <link rel="stylesheet" href="assets/css/style.css?v=20260519c">
+    <link rel="stylesheet" href="assets/css/style.css?v=20260520a">
     <!-- QuaggaJS for barcode scanning -->
     <script src="https://cdn.jsdelivr.net/npm/@ericblade/quagga2@1.8.4/dist/quagga.min.js"></script>
     <!-- @xenova/transformers: ES-module bootstrap that exposes a lazy category-classifier as window._categoryPipelinePromise -->
@@ -54,8 +54,11 @@
 <div id="app-preloader" aria-hidden="true">
     <div class="app-preloader-inner">
         <img src="assets/img/logo/logo.png" alt="EverShelf" class="app-preloader-logo" />
-        <div class="app-preloader-spinner"></div>
-        <span class="app-preloader-version" id="preloader-version">v1.7.15</span>
+        <div class="app-preloader-spinner" id="preloader-spinner"></div>
+        <div id="preloader-checks" class="preloader-checks" style="display:none"></div>
+        <div id="preloader-error-msg" class="preloader-error-msg" style="display:none"></div>
+        <button id="preloader-retry-btn" class="preloader-retry-btn" style="display:none" onclick="_startupRetry()">🔄 <span data-i18n="startup.retry">Riprova</span></button>
+        <span class="app-preloader-version" id="preloader-version">v1.7.20</span>
     </div>
 </div>
 
@@ -68,7 +71,7 @@
         <!-- Title — left-aligned; grows to fill space -->
         <div class="header-title-wrap">
             <h1 class="header-title" onclick="showPage('dashboard')">
-                <img src="assets/img/logo/logo_icon.png" alt="" class="header-logo-icon" aria-hidden="true" /><span data-i18n="nav.title">EverShelf</span><span class="header-version">v1.7.15</span>
+                <img src="assets/img/logo/logo_icon.png" alt="" class="header-logo-icon" aria-hidden="true" /><span data-i18n="nav.title">EverShelf</span><span class="header-version">v1.7.20</span>
             </h1>
             <!-- Update badge — shown alongside title, never replaces it -->
             <span class="header-update-badge" id="header-update-badge" style="display:none"></span>
@@ -1604,6 +1607,6 @@
     </div>
 </div>
 
-<script src="assets/js/app.js?v=20260519c"></script>
+<script src="assets/js/app.js?v=20260520a"></script>
 </body>
 </html>