docs(excalidraw,roadmap): #78 H1-H3 doc utilisateur + #61/#74 optionnels non retenus

- README FR/EN : .excalidraw dans les formats supportes + section Diagrammes Excalidraw
- Guide integre : nouvelle section Excalidraw (i18n FR/EN) + note compatibilite plugin Obsidian
- ROADMAP : #78 termine (B5/F3/H1-H3), C8/F2 non retenus ; #61 et #74 optionnels marques non retenus
- CHANGELOG : entree Documentation

frontend/locales/en.json

@@ -1076,6 +1076,7 @@
   "help.nav_tips": "💡 Tips",
   "help.nav_webhooks": "🪝 Webhooks",
   "help.nav_plugins": "🧩 Plugins",
+  "help.nav_excalidraw": "🎨 Excalidraw",
   "help.new_file": "New file",
   "help.new_folder": "New folder",
   "help.nights": "sleepless nights 😅",
@@ -1668,5 +1669,15 @@
       "help.plugins_security_3": "Secure packaging: the .zip is checked against path traversal on install.",
       "help.plugins_security_4": "Admin only: installation and removal require the admin role.",
       "help.plugins_template": "Use the <strong>Template</strong> button in Settings > Plugins to generate a valid starter plugin.",
+      "help.excalidraw_title": "🎨 Excalidraw diagrams",
+      "help.excalidraw_intro": "<code>.excalidraw</code> and <code>.excalidraw.md</code> files open in a full visual Excalidraw editor embedded in a sandboxed iframe.",
+      "help.excalidraw_open_title": "Opening",
+      "help.excalidraw_open": "Click a <code>.excalidraw</code> file in the tree: the editor opens instead of the markdown viewer.",
+      "help.excalidraw_edit": "Draw, add shapes, text, arrows and images — the complete Excalidraw experience.",
+      "help.excalidraw_save_title": "Saving",
+      "help.excalidraw_save": "Changes are saved automatically (2s debounce) or with <strong>Ctrl+S</strong>. The editor follows the light/dark theme.",
+      "help.excalidraw_search_title": "Search",
+      "help.excalidraw_search": "Text inside diagram elements is extracted on indexing, so it is searchable via the full-text search.",
+      "help.excalidraw_compat": "Files created with the Obsidian Excalidraw plugin (including the compressed <code>.excalidraw.md</code> format) are compatible.",
       "help.footer_tagline": "- Web gateway for your Obsidian vaults"
   }