fix: avoid panic when log file cannot be created

[kinderjoypresents]

Summary

This PR prevents mega-evme from panicking when --log.file points to an invalid/unwritable path.

Changes

• Replace expect() on std::fs::File::create with graceful error handling.
• On failure, print a short message to stderr and fall back to stderr logging.

Motivation

Passing an invalid --log.file path should not crash the CLI; it should continue running with stderr logging.

[Troublor]

LGTM, thanks!

[kinderjoypresents]

LGTM, thanks!

Do I need to update the file so you can merge it?

[Troublor]

@kinderjoypresents The CI workflow is currently misconfigured for external PRs. We will fix that soon.