feat: soft-delete and undo/restore for local-workspace items

Backend:
- DELETE /api/local-workspace/items/{id} now soft-deletes (sets deleted_at
  instead of hard DELETE) — items go to trash, not permanently gone
- New POST /api/local-workspace/items/{id}/restore to undo a delete
- _load_workspace_pages() and _load_children() now filter deleted_at IS NULL
  so soft-deleted items disappear from sidebar and tree queries

Frontend:
- undoDelete() now calls the restore API per item (single or bulk)
- bulkDelete() now shows the same undo banner as context menu delete
- All delete paths (ctxMenu, bulk, modal) use the same soft-delete + undo flow

app/routers/board.py

@@ -172,7 +172,7 @@ def _load_workspace_pages(ws_cookie: str) -> list:
         ws_id = int(ws_cookie)
         with get_conn() as conn:
             rows = conn.execute(
-                "SELECT id, title, parent_section, content_format FROM pages WHERE workspace_id=? AND parent_id IS NULL ORDER BY created_at DESC",
+                "SELECT id, title, parent_section, content_format FROM pages WHERE workspace_id=? AND parent_id IS NULL AND deleted_at IS NULL ORDER BY created_at DESC",
                 (ws_id,),
             ).fetchall()
         items = []
@@ -197,7 +197,7 @@ def _load_children(parent_id: int) -> list:
     """Recursively load children of a page."""
     with get_conn() as conn:
         rows = conn.execute(
-            "SELECT id, title, parent_section, content_format FROM pages WHERE parent_id=? ORDER BY created_at",
+            "SELECT id, title, parent_section, content_format FROM pages WHERE parent_id=? AND deleted_at IS NULL ORDER BY created_at",
             (parent_id,),
         ).fetchall()
     children = []