feat: add pre-run pipeline validation warnings

[xxivani]

Description

Adds a pre-run pipeline validation pass that checks for known operator incompatibilities before sending the pipeline to the backend. A new validatePipeline utility walks the pipeline steps tracking channel state (colour vs grayscale) and flags operators that will likely fail given their input. Warnings are shown in a PipelineWarningsModal with a Cancel / Run Anyway choice so the check is never a hard blocker.
In reference to pr #198

Fixes #197

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

How Has This Been Tested?

Tested the following cases manually: grayscale operator into a colour-only block, colour image into a grayscale-only block, missing Read Image block, and a valid pipeline with no warnings (runs immediately without showing the modal).

• Existing tests pass
• New tests added
• Manual testing

Screenshots (if applicable)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review
• I have added/updated documentation as needed
• My changes generate no new warnings
• Tests pass locally

[kx7m2qd]

Reviewed the validatePipeline logic the channel state tracking approach (colour vs grayscale) looks clean. One thought: what happens if the pipeline has zero steps? Does validatePipeline return empty warnings and skip the modal entirely, or does it still show it? Also noticed the branch is out of date with main — might be worth a rebase before merge to avoid surprises.