feat(boot): écran de chargement inline + staging backend/frontend pour le bundle desktop

- frontend/index.html : splash #boot-splash 100% inline (CSS+JS), scripts CDN en defer pour ne pas bloquer le premier paint
- frontend/js/app.js : signale le boot terminé via __OBSIGATE_BOOTED (try/finally)
- desktop/src/main.rs : met à jour le statut du splash avant la redirection vers le backend
- ci: stage backend/ + frontend/ sous desktop/ avant tauri build (resources backend/** et frontend/**) dans desktop-build.yml et build-linux.sh
- desktop/gen/schemas/windows-schema.json : régénéré (permissions store/updater)
- docs: README, README.fr, ROADMAP

frontend/index.html

@@ -47,6 +47,33 @@
             href="/static/icons/icon-192x192.svg"
         />
 
+        <!-- Boot splash CSS — inline so it applies BEFORE /static/style.css is
+             available (desktop: embedded page served from tauri:// has no /static) -->
+        <style id="boot-splash-css">
+            body:not(.app-booted) > *:not(#boot-splash) { display: none !important; }
+            #boot-splash {
+                position: fixed; inset: 0; z-index: 2147483647;
+                display: flex; align-items: center; justify-content: center;
+                background: radial-gradient(1200px 600px at 50% -10%, #1b2030 0%, #0e1117 60%);
+                color: #e6e8ee; font-family: "Segoe UI", system-ui, -apple-system, sans-serif;
+            }
+            body.app-booted #boot-splash { display: none; }
+            .bs-card { display: flex; flex-direction: column; align-items: center; gap: 14px; }
+            .bs-logo {
+                width: 72px; height: 72px; display: flex; align-items: center; justify-content: center;
+                font-size: 38px; background: #171c26; border: 1px solid #2a3242; border-radius: 18px;
+                box-shadow: 0 8px 30px rgba(0, 0, 0, 0.45);
+            }
+            .bs-title { font-size: 22px; font-weight: 600; letter-spacing: 0.3px; }
+            .bs-spinner {
+                width: 34px; height: 34px; border-radius: 50%;
+                border: 3px solid rgba(124, 58, 237, 0.25); border-top-color: #7c3aed;
+                animation: bs-spin 0.9s linear infinite;
+            }
+            @keyframes bs-spin { to { transform: rotate(360deg); } }
+            .bs-status { font-size: 13px; color: #9aa3b2; min-height: 18px; }
+            @media (prefers-reduced-motion: reduce) { .bs-spinner { animation-duration: 1.8s; } }
+        </style>
         <link rel="stylesheet" href="/static/style.css" />
         <link
             rel="stylesheet"
@@ -59,9 +86,10 @@
             id="hljs-theme-light"
             disabled
         />
-        <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/highlight.min.js"></script>
-        <script src="https://unpkg.com/[email protected]/dist/umd/lucide.min.js"></script>
-        <script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
+        <!-- defer: ne bloque pas le premier paint (le splash inline s'affiche immédiatement) -->
+        <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/highlight.min.js"></script>
+        <script defer src="https://unpkg.com/[email protected]/dist/umd/lucide.min.js"></script>
+        <script defer src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
         <script type="importmap">
             {
                 "imports": {
@@ -172,6 +200,44 @@
         </script>
     </head>
     <body>
+        <!-- Boot splash : visible tant que l'app n'a pas booté avec son CSS.
+             Sur desktop, la page embarquée (tauri://) n'a pas /static/style.css →
+             sans ce splash, le squelette HTML brut serait visible pendant le
+             démarrage du backend (FOUC 3-10 s). Styles 100% inline. -->
+        <div id="boot-splash" role="status" aria-live="polite">
+            <div class="bs-card">
+                <div class="bs-logo">📖</div>
+                <div class="bs-title">ObsiGate</div>
+                <div class="bs-spinner" aria-hidden="true"></div>
+                <div class="bs-status" id="boot-splash-status">Démarrage…</div>
+            </div>
+        </div>
+        <script>
+            (function () {
+                var done = false;
+                function bootReady() {
+                    if (done) return;
+                    done = true;
+                    document.body.classList.add("app-booted");
+                    var s = document.getElementById("boot-splash");
+                    if (s && s.parentNode) s.parentNode.removeChild(s);
+                }
+                window.__obsigateBootReady = bootReady;
+                window.__setBootStatus = function (txt) {
+                    var el = document.getElementById("boot-splash-status");
+                    if (el) el.textContent = txt;
+                };
+                var poll = setInterval(function () {
+                    if (window.__OBSIGATE_BOOTED) { clearInterval(poll); bootReady(); }
+                }, 80);
+                // Filet de sécurité : si le CSS principal a chargé (origine backend réelle)
+                // mais que le JS a échoué, révéler l'app après 10 s au lieu d'un splash infini.
+                setTimeout(function () {
+                    var link = document.querySelector('link[href*="style.css"]');
+                    if (link && link.sheet) { clearInterval(poll); bootReady(); }
+                }, 10000);
+            })();
+        </script>
         <div class="search-progress-bar" id="search-progress-bar">
             <div class="search-progress-bar__fill"></div>
         </div>

frontend/js/app.js

@@ -102,8 +102,15 @@ async function init() {
 }
 
 document.addEventListener("DOMContentLoaded", () => {
-  init();
-  Sync.init();
+  try {
+    init();
+    Sync.init();
+  } finally {
+    // Signale au splash de démarrage (index.html #boot-splash) que l'app est
+    // initialisée : le splash est retiré et le squelette stylé révélé.
+    window.__OBSIGATE_BOOTED = true;
+    if (typeof window.__obsigateBootReady === "function") window.__obsigateBootReady();
+  }
   // Show version in header
   fetch('/api/health')
     .then(r => r.json())