Communicate path character limits in GUI tools

[Jayant-kernel]

Fixes QubesOS/qubes-issues#6943.

What changed

• Added a shared translated message for path character limits used by qubes.SelectFile and qubes.SelectDirectory.
• Made get_path_from_vm() raise that message for non-ASCII paths and other unsupported characters.
• Added the same path-limit hint as a tooltip in Backup and Boot Qube From Device.
• Updated Backup and Boot Qube From Device to show the clearer warning when remote path selection returns unsupported characters.
• Added focused tests for accepted paths, invalid paths, Backup warning handling, and Boot Qube From Device tooltip/warning coverage.

Validation

• python -m py_compile qubesmanager\utils.py qubesmanager\backup_utils.py qubesmanager\backup.py qubesmanager\bootfromdevice.py qubesmanager\tests\test_utils.py qubesmanager\tests\test_backup_utils.py qubesmanager\tests\test_bootfromdevice.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.86%. Comparing base (efff1ef) to head (2f2f4b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
==========================================
+ Coverage   69.73%   70.86%   +1.13%     
==========================================
  Files          17       17              
  Lines        4015     4030      +15     
==========================================
+ Hits         2800     2856      +56     
+ Misses       1215     1174      -41     

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[Jayant-kernel]

@marmarta
review the changes , once free

[marmarta]

Have you tested this? I tried to run it on a test system and I have not seen any effect (namely, VM settings still fails silently and crashes to death if I manually input a wrong path, backup allows me to proceed and then tells me it couldn't write the backup to file).