mesg: move uu_app() to bottom of file

[cakebaker]

Currently, uu_app() is sandwiched between two uumain() function. To "un-sandwich" it, this PR moves uu_app() to the bottom of the file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (1dcb552) to head (48bdb19).
⚠️ Report is 85 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #304   +/-   ##
===========================
===========================

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