fix(dashboard): keep shell during menu transitions

Replace document navigation with native Streamlit state transitions, preserve dashboard contracts, and add regression plus authenticated browser evidence for Issue #9.

dashboard/app.py

@@ -12,7 +12,15 @@ if __package__ in (None, ""):
 
 st.set_page_config(page_title="Jarca Ledger", page_icon="💰", layout="wide")
 
-from dashboard.common import clear_cache, get_db_path, get_transfer_review_items, inject_app_css, render_sidebar_nav  # noqa: F401
+from dashboard.common import (
+    clear_cache,
+    get_db_path,
+    get_transfer_review_items,  # noqa: F401
+    inject_app_css,
+    render_dashboard_body,
+    render_sidebar_nav,
+    validate_dashboard_page,
+)  # noqa: F401
 from dashboard.authentication import require_dashboard_authentication
 from dashboard.page_views.accounts import page_accounts
 from dashboard.page_views.assets import page_assets
@@ -32,15 +40,15 @@ def main() -> None:
     db_path = get_db_path()
     inject_app_css()
     pages = ["월정산", "요약", "자산/마감", "거래내역", "확인필요 큐", "미분류", "분류 규칙", "카테고리", "계좌", "Import"]
-    query_page = st.query_params.get("page", "요약")
-    page = query_page if query_page in pages else "요약"
-    st.session_state["page"] = page
+    query_page = st.query_params.get("page")
+    page = validate_dashboard_page(query_page, pages)
+    st.session_state["dashboard_page"] = page
 
     with st.sidebar:
         st.markdown("## 💰 Jarca Ledger")
         st.caption("가계부 · 분류 · Import 운영 콘솔")
         st.markdown('<div class="jarca-nav-title">메뉴</div>', unsafe_allow_html=True)
-        render_sidebar_nav(pages, page)
+        page = render_sidebar_nav(pages, page)
         if st.button("새로고침", key="refresh_button", use_container_width=True):
             clear_cache()
             st.rerun()
@@ -56,7 +64,7 @@ def main() -> None:
         "계좌": page_accounts,
         "Import": page_imports,
     }
-    dispatch[page](db_path)
+    render_dashboard_body(page, lambda: dispatch[page](db_path))
 
 
 if __name__ == "__main__":