PowerShell splits `-o=file.xlsx` into `-o=file` + `.xlsx` as separate
args. Fix: auto-append .xlsx if output has no extension, and filter
out .xlsx files from input args with a helpful warning.
- Add --version flag to CLI (closes bd-bxt)
- Fix ldflags module path: csv2excel/internal/version → code.beautifulmachines.dev/jakoubek/csv2excel/internal/version
so build info (version, commit, date) is actually embedded into the binary
- Add mg.Deps(Build) to Install so it always builds before installing
