fix(nav): preserve POI checkbox order

[xiaolefang-dm]

Summary

• preserve POI checkbox selection order in the Operate tab
• show selected order next to each checked POI name
• re-index backend POI payload as a dense queue so downstream navigation follows the UI order

Checks

• python AST parse for app/backend/node_manager.py
• dart analyze app/frontend/lib/pages/operate_tab.dart (existing deprecation infos only)

Screen.Recording.2026-05-13.at.20.15.14.mov

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (240.3 files/s, 35826.5 lines/s)

Language	files	blank	comment	code
Python	13	620	262	3464
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	31	30	202
JSON	2	8	0	58
TOML	1	4	0	58
make	1	18	15	55
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	941	360	4961