feat: collaboration temps reel - edition simultanee (#62)

WebSocket /ws/collab/{vault}/{path} (rooms par fichier), relais Yjs/CRDT, awareness (curseurs colores + presence), persistance serveur debounce 2s, auth WS + check_vault_access, reconnexion automatique. Frontend frontend/js/collab.js, backend backend/collab.py. Tests: 17 backend (5 clients simultanes) + 10 frontend. Docs: CHANGELOG, ROADMAP, fiche features/collaboration.md, README FR/EN.

frontend/index.html

@@ -93,7 +93,8 @@
         <script type="importmap">
             {
                 "imports": {
-                    "@codemirror/state": "https://esm.sh/@codemirror/[email protected]"
+                    "@codemirror/state": "https://esm.sh/@codemirror/[email protected]",
+                    "yjs": "https://esm.sh/[email protected]"
                 }
             }
         </script>
@@ -111,8 +112,11 @@
                 rectangularSelection,
                 crosshairCursor,
                 highlightActiveLine,
+                Decoration,
+                ViewPlugin,
+                WidgetType,
             } from "https://esm.sh/@codemirror/[email protected]?external=@codemirror/state";
-            import { EditorState } from "@codemirror/state";
+            import { EditorState, StateEffect } from "@codemirror/state";
             import {
                 defaultHighlightStyle,
                 syntaxHighlighting,
@@ -196,6 +200,10 @@
                 rust,
                 oneDark,
                 keymap,
+                Decoration,
+                ViewPlugin,
+                WidgetType,
+                StateEffect,
             };
         </script>
     </head>
@@ -1368,6 +1376,8 @@
                         title="Renommer le fichier"
                     />
                     <div class="editor-spacer"></div>
+                    <div class="collab-presence" id="collab-presence" style="display: none"
+                        title="Collaboration"></div>
                     <span class="editor-save-dot ok" id="editor-save-dot"
                         ><span class="dot"></span
                         ><span id="editor-save-label">Saved</span></span