fix: prevent cross-user workspace leak via stale cookie

- _get_active_workspace now verifies workspace owner matches current user
- Fallback: only picks workspace owned by current user (not any user)
- /local-workspace redirects to /workspaces when no workspace exists
- New users no longer see other users' workspaces/projects

app/routers/dashboard.py

@@ -581,9 +581,13 @@ async def local_workspace_page(request: Request, folder: int = None):
     if not user:
         return RedirectResponse("/auth/login?provider=local", status_code=302)
 
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=user["id"])
     ws_id = ws["id"] if ws else None
 
+    # If no workspace exists for this user, redirect to workspaces page
+    if not ws_id:
+        return RedirectResponse("/workspaces", status_code=302)
+
     # Build breadcrumb if navigating into a folder
     breadcrumb = []
     current_folder_id = folder
@@ -617,7 +621,7 @@ async def local_workspace_tree(request: Request, folder: int = None):
     If ?folder=ID is provided, returns only that folder's children.
     Otherwise returns the full recursive tree from root.
     """
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=_get_user_id(request))
     ws_id = ws["id"] if ws else None
     if not ws_id:
         return {"tree": [], "breadcrumb": []}
@@ -777,7 +781,7 @@ async def create_local_workspace_item(request: Request):
     item_type = body.get("type", "page")
     parent_id = body.get("parent_id")
     explicit_ws_id = body.get("workspace_id")
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=_get_user_id(request))
     ws_id = explicit_ws_id or (ws["id"] if ws else None)
     ws_key = ws["workspace_key"] if ws and not explicit_ws_id else ""
     section = 'Workspace' if item_type == 'folder' else 'Private'
@@ -867,7 +871,7 @@ async def upload_local_workspace_file(request: Request):
     import json
     from pathlib import Path
 
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=_get_user_id(request))
     ws_id = ws["id"] if ws else None
     if not ws_id:
         return JSONResponse({"error": "No active workspace"}, status_code=400)
@@ -940,7 +944,7 @@ async def upload_local_workspace_folder(request: Request):
     import json
     from pathlib import Path
 
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=_get_user_id(request))
     ws_id = ws["id"] if ws else None
     if not ws_id:
         return JSONResponse({"error": "No active workspace"}, status_code=400)
@@ -1043,22 +1047,30 @@ async def upload_local_workspace_folder(request: Request):
 WORKSPACE_COOKIE = "flowdeck_workspace"
 
 
-def _get_active_workspace(request: Request) -> dict | None:
-    """Get the active workspace ID from the cookie, or first workspace, or None."""
+def _get_active_workspace(request: Request, user_id: int = None) -> dict | None:
+    """Get the active workspace ID from the cookie (verified for current user), or first user workspace, or None."""
     ws_id = request.cookies.get(WORKSPACE_COOKIE)
     if ws_id:
         try:
             with get_conn() as conn:
-                ws = conn.execute("SELECT * FROM workspaces WHERE id=?", (int(ws_id),)).fetchone()
+                ws = conn.execute("SELECT * FROM workspaces WHERE id=?",
+                                  (int(ws_id),)).fetchone()
                 if ws:
-                    return dict(ws)
+                    ws_dict = dict(ws)
+                    # Verify ownership — only return if it belongs to the current user
+                    if user_id is None or ws_dict.get("owner_id") == user_id:
+                        return ws_dict
         except (ValueError, Exception):
             pass
-    # Fallback: first workspace
-    with get_conn() as conn:
-        ws = conn.execute("SELECT * FROM workspaces ORDER BY id LIMIT 1").fetchone()
-        if ws:
-            return dict(ws)
+    # Fallback: first workspace owned by this user
+    if user_id:
+        with get_conn() as conn:
+            ws = conn.execute(
+                "SELECT * FROM workspaces WHERE owner_id=? ORDER BY id LIMIT 1",
+                (user_id,)
+            ).fetchone()
+            if ws:
+                return dict(ws)
     return None
 
 
@@ -1091,7 +1103,7 @@ async def list_workspaces(request: Request):
     for r in rows:
         d = dict(r)
         workspaces.append(d)
-    active = _get_active_workspace(request)
+    active = _get_active_workspace(request, user_id=_get_user_id(request))
     return {"workspaces": workspaces, "active_id": active["id"] if active else None}
 
 
@@ -1310,7 +1322,7 @@ async def list_all_tags_global(request: Request):
 @router.get("/api/local-workspace/tags")
 async def list_tags(request: Request):
     """List tags used in the active workspace (current user's tags only)."""
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=_get_user_id(request))
     ws_id = ws["id"] if ws else None
     uid = _get_user_id(request)
     if not ws_id:
@@ -1392,7 +1404,7 @@ async def remove_item_tag(item_id: int, tag_id: int):
 @router.get("/api/local-workspace/tags/search")
 async def search_by_tags(request: Request, tags: str = ""):
     """Search items by tags (comma-separated)."""
-    ws = _get_active_workspace(request)
+    ws = _get_active_workspace(request, user_id=_get_user_id(request))
     ws_id = ws["id"] if ws else None
     if not ws_id:
         return {"items": []}