Add basic filters: Gaussian blur, median, sharpen

[yangfei]

Summary

Add common basic filters to the Process menu.

Scope

• Implement Gaussian blur, median filter, and sharpen in core code.
• Add menu commands in the app.
• Keep parameter handling simple for the first version.
• Add tests using small deterministic images.

Acceptance criteria

• User can apply Gaussian blur.
• User can apply median filter.
• User can apply sharpen.
• Core algorithms have automated tests.
• Existing processing commands still work.