fix(pdf,excalidraw): BUG-001 + BUG-002

BUG-001 (PDF stream 500) : le nom Unicode du fichier etait injecte brut
dans Content-Disposition -> en-tete HTTP invalide -> 500. Nouveau helper
_content_disposition (RFC 5987 : filename ASCII + filename*=UTF-8'').
Applique aux 2 branches /pdf/stream. Test: tests/test_pdf_stream.py.

BUG-002 (Excalidraw 'Loading...' infini) : deux causes.
1) L'import esm.sh avec ?alias=react:... etait propage a chaque dep
   transitive; esm.sh renvoie 408 sur les builds a gamme semver (jotai@>=2.9.2)
   -> import entier echoue. On retire l'alias et on utilise React 19 coherent
   (spec a gamme + target identique a celle d'Excalidraw) -> plus de 408.
2) L'editeur passait la prop legacy 'excalidrawRef', inoperante en 0.18
   (la prop reelle est 'excalidrawAPI') -> l'API n'etait jamais recue,
   le 'Loading' ne se masquait pas, save/export morts.

Verifie : 534 tests backend verts (dont 3 nouveaux PDF) + E2E navigateur
(loading masque + cycle requestSave->save OK).

frontend/excalidraw-editor.html

@@ -92,26 +92,20 @@
     <button id="btn-export-svg" title="Export SVG">📐 SVG</button>
   </div>
 
-  <!-- Import map: force ALL modules (Excalidraw + sub-deps) to use React 18.3.1 -->
-  <script type="importmap">
-  {
-    "imports": {
-      "react": "https://esm.sh/[email protected]",
-      "react-dom": "https://esm.sh/re[email protected]",
-      "react-dom/": "https://esm.sh/[email protected]/"
-    }
-  }
-  </script>
-
-  <!-- Single module script -->
+  <!-- Excalidraw is loaded from esm.sh WITHOUT the `?alias=react:…` query.
+       The alias gets propagated by esm.sh to every transitive dependency; for
+       range-version deps (e.g. jotai@>=2.9.2) esm.sh then tries to rebuild the
+       range + alias variant and times out (HTTP 408) → the whole import fails →
+       the diagram stays stuck on "Loading Excalidraw…". Dropping the alias lets
+       esm.sh resolve a consistent React 19 (Excalidraw 0.18's default), which
+       loads reliably. -->
   <script type="module">
-    import React from "react";
-    import ReactDOM from "react-dom/client";
+    import React from "https://esm.sh/react@^19.2.0?target=es2022";
+    import { createRoot } from "https://esm.sh/react-dom@^19.2.0/client?target=es2022";
     import { decompressFromBase64 } from "https://esm.sh/[email protected]";
-    import * as ExcalidrawLib from "https://esm.sh/@excalidraw/[email protected]?alias=react:[email protected],react-dom:[email protected]";
+    import * as ExcalidrawLib from "https://esm.sh/@excalidraw/[email protected]";
 
     window.React = React;
-    window.ReactDOM = ReactDOM;
     window.EXCALIDRAW_ASSET_PATH = "https://esm.sh/@excalidraw/[email protected]/dist/prod/";
     window.ExcalidrawLib = ExcalidrawLib;
 
@@ -186,7 +180,11 @@
     function App({ initialData, theme }) {
       const [appState, setAppState] = React.useState(null);
 
-      const excalidrawRef = React.useCallback((api) => {
+      // Excalidraw 0.18 exposes its imperative API through the `excalidrawAPI`
+      // prop, called with the API object once mounted (NOT the legacy
+      // `excalidrawRef` name). Without this the loading overlay never hides
+      // and save/export stay dead.
+      const onReady = React.useCallback((api) => {
         if (api) {
           excalidrawAPI = api;
           loadingEl.classList.add("hidden");
@@ -201,7 +199,7 @@
       }, []);
 
       return React.createElement(Excalidraw, {
-        excalidrawRef: excalidrawRef,
+        excalidrawAPI: onReady,
         initialData: initialData,
         onChange: onChange,
         theme: theme,
@@ -247,10 +245,10 @@
             setTheme(currentTheme);
 
             try {
-              if (typeof ReactDOM.createRoot !== "function") {
-                throw new Error("ReactDOM.createRoot is not available — check react-dom/client import");
+              if (typeof createRoot !== "function") {
+                throw new Error("createRoot is not available — check react-dom/client import");
               }
-              ReactDOM.createRoot(document.getElementById("root")).render(
+              createRoot(document.getElementById("root")).render(
                 React.createElement(App, {
                   initialData: initialData,
                   theme: currentTheme,