fix: use BoxFit.cover for camera preview to fill panel

[xiaolefang-dm]

• Change camera preview from BoxFit.contain (letterboxed) to BoxFit.cover (fills panel with center crop)
• Fullscreen preview still uses BoxFit.contain to show full image
• Fixes black bars on video streams with different aspect ratios

[github-actions]

cloc	github.com/AlDanial/cloc v 2.04 T=0.17 s (252.9 files/s, 37191.0 lines/s)

Language	files	blank	comment	code
Python	13	613	262	3399
Markdown	4	155	6	426
C++	4	60	38	409
Bourne Shell	15	41	9	272
Dockerfile	1	30	29	191
JSON	2	8	0	58
TOML	1	4	0	57
make	1	18	15	55
CMake	1	4	0	17
--------	--------	--------	--------	--------
SUM:	42	933	359	4884