fix(cli): reject .wep extension alias

[ubugeeei]

Summary

• keep .webp support
• reject the nonstandard .wep alias in image inference and CLI format parsing
• add regression tests for both surfaces

Closes #5

Validation

• cargo fmt --package fastqr-image --package fastqr-tui
• cargo test -p fastqr-image
• cargo test -p fastqr-tui
• git diff --check

[ubugeeei]

Landed via #27 after local vp run ci and final GitHub Actions CI passed. Closing this issue-scoped PR to keep the queue and PR list clean.