refactor: correctly specify static lifetime for returned error

[Enduriel]

This previously made it very difficult to bubble up returned errors

[Copilot]

Pull Request Overview

Refactors error return types to use static lifetime instead of borrowing from input parameters, making error propagation easier by removing lifetime dependencies.

• Changes error return type from &'a str to &'static str in two from_image methods
• Eliminates the need for error values to be tied to the input parameter's lifetime

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/encoder.rs	Updates Encoder::from_image error return type to use static lifetime
src/animation_encoder.rs	Updates AnimFrame::from_image error return type to use static lifetime

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}