box: add webcam input and fix bash conditionals

[rickstaa]

Summary

• Add INPUT_WEBCAM env var to stream.sh for webcam input via v4l2 (e.g. /dev/video0)
• Document video file and webcam input options in box.md
• Fix broken [ ... && ... ] conditional in box.sh (caused [: missing ] error) and switch all conditionals to bash [[ ]] for consistency

Test plan

• Run make box REBUILD=false DOCKER=true and verify no syntax errors
• Test INPUT_WEBCAM=/dev/video0 make box-stream with a webcam
• Test INPUT_VIDEO=/path/to/file.mp4 make box-stream with a video file
• Test default (no input vars) still uses test pattern

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.77913%. Comparing base (ad7d5e4) to head (bb00d7b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3882         +/-   ##
===================================================
- Coverage   32.81243%   32.77913%   -0.03330%     
===================================================
  Files            171         171                 
  Lines          42042       42042                 
===================================================
- Hits           13795       13781         -14     
- Misses         27212       27227         +15     
+ Partials        1035        1034          -1     

see 3 files with indirect coverage changes

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