auto-generate output filename when not specified

[rootvector2]

Fixes #3090

Allow avifenc to auto-generate the output filename when only a single input file is provided and no output is specified. The output name is derived by replacing the input extension with .avif, or appending .avif if the input has no extension.

This behavior only applies when exactly one input file is given and no -o or positional output argument is present, so existing CLI behavior remains unchanged.

Also adds a safeguard to prevent accidentally overwriting the input file when the input already has a .avif extension.

[rootvector2]

This PR focuses only on the optional output filename behavior. The other suggestions in the issue (multi-input batch conversion and wildcard handling) may require maintainer guidance since multiple inputs currently have existing semantics (e.g., animations).