fix(upgrade): skip Modrinth versions with empty files

[SAY-5]

Fixes #514.

Problem

When ferium upgrade fetches the Modrinth version list for a project, some legacy/broken entries can come back with an empty files array (e.g. EasyAuth, Silk in the issue thread). from_mr_version then calls VersionExt::get_version_file, which does .unwrap_or(&self.files[0]) and panics:

thread 'ferium-worker' panicked at libium/src/version_ext.rs:15:35:
index out of bounds: the len is 0 but the index is 0

Fix

Filter empty-files versions out of version_list() results in mod_downloadable.rs before mapping with from_mr_version. They are skipped the same way they would be if no version satisfied the filters, instead of crashing the worker.

Mirrors the no-files-available handling added for Modrinth modpacks in #508.

Testing

cargo build -p libium --lib and cargo clippy -p libium --lib --no-deps -- -D warnings pass cleanly. cargo test -p libium --lib runs (no existing unit tests in libium).

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