cli : load backends after validating input files

[erikdw]

I noticed that expensive operations (e.g., compiling shaders) are done prior to checking if the input file even exists.

Solution:

• Move backend loading after the input-file checks so we can fail early.
• Also fail early if the model path is missing.

Note

AI assistant used to locate the call and apply the move. I can discuss the change.

Made with Cursor

[danbev]

The failing android CI jobs were fixed as part of 4afa009.