fix(map): infer preview POI z from SDF column

[xiaolefang-dm]

Summary

• infer newly added map preview POI z from the map's sdf_map.npy and occupancy_grid.npy
• keep the clicked map x/y unchanged while choosing the nearest odom-seeded SDF height in that x/y column
• avoid occupied voxels when selecting the z layer, falling back to the submitted z if map data cannot be read

1.mov

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (250.7 files/s, 37049.9 lines/s)

Language	files	blank	comment	code
Python	13	615	262	3414
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	936	360	4911

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (247.7 files/s, 36611.5 lines/s)

Language	files	blank	comment	code
Python	13	615	262	3414
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	936	360	4911

[xiaolefang-dm]

Updated in 38c2d76. I removed the silent fallback to the submitted z: if the map SDF/occupancy/meta files cannot be loaded, shapes are inconsistent, the clicked x/y is outside the map, or no valid non-occupied SDF voxel exists in that column, POI creation now fails with an HTTP error instead of writing a potentially invalid POI.