fix: sidebar workspace section — hide create buttons quand aucun workspace actif

- Ajout flag has_active_workspace dans _sidebar_data (dashboard.py + board.py)
- Template base.html: quand has_active_workspace=False:
  - Titre section → '📁 No workspace open'
  - Boutons New File/New Folder cachés
  - Message 'Open a workspace to see your files'
- Guard JS newPageInWorkspace()/newFolderInWorkspace(): toast + redirect si pas de workspace
- 143 tests passent

app/routers/board.py

@@ -229,6 +229,7 @@ def _sidebar_data(request: Request, owner: str = "", repo: str = "") -> dict:
     gitea_workspace = False
     gitea_owner = ""
     gitea_repo = ""
+    has_active_workspace = False
 
     if ws_cookie and ws_cookie.startswith("gitea:"):
         # Gitea workspace: preserve context across pages
@@ -238,6 +239,7 @@ def _sidebar_data(request: Request, owner: str = "", repo: str = "") -> dict:
             gitea_repo = parts[2]
             active_ws_name = f"{gitea_owner}/{gitea_repo}"
             gitea_workspace = True
+            has_active_workspace = True
             workspace_pages = []  # loaded client-side
             # Get local workspace ID for mirror
             if user:
@@ -262,6 +264,7 @@ def _sidebar_data(request: Request, owner: str = "", repo: str = "") -> dict:
                 if row:
                     active_ws_name = row["name"]
                     workspace_pages = _load_workspace_pages(ws_cookie)
+                    has_active_workspace = True
         except (ValueError, Exception):
             pass
     local_ws_id = 0
@@ -376,7 +379,8 @@ def _sidebar_data(request: Request, owner: str = "", repo: str = "") -> dict:
             "user": user,
             "auth_method": auth_method,
             "gitea_linked": gitea_linked,
-            "github_linked": github_linked}
+            "github_linked": github_linked,
+            "has_active_workspace": has_active_workspace}
 
 
 def _extract_ai_keywords(owner: str, repo: str, labels: list[dict], body: str = ""):