fix default icon path interpolation

[zhonggang666]

What this PR does / why we need it?

Fixes defaultIcon so window.MaxKB.prefix is interpolated instead of treated as a literal string.

Closes #6369.

Summary of your change

Changes the default icon path from a normal quoted string to a template literal.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

Validation: ran git diff --check.

Note: Frontend type-check was not run because this checkout does not include ui/node_modules or a frontend lockfile.