Fix broken Markdown image assets

[Ziwen-Lii]

Summary

• Fix two screenshot references in the ima knowledge-base Case that used Windows path separators and therefore did not resolve after deployment.
• Give the screenshots distinct, descriptive alternative text.
• Add a Vitest regression check that verifies every local Markdown image uses POSIX separators and resolves to an existing file, including percent-encoded filenames.

Validation

• Ran a Node-based equivalent of the new asset check across all 51 Markdown files; all local image references resolved.
• Ran git diff --check.
• Attempted npm run test and npm run docs:build, but the local package registry did not complete dependency installation in this environment; CI can run the new Vitest check with the project dependencies available.