fix(navigator): refine symbol scope highlighting and UI adjustments

[spelech]

Summary

• Refines symbol scope highlighting to use a subtle ambient tint on enclosing AST symbol body lines while keeping the primary declaration line distinctly accented.
• Adds transparent border-left base on code viewer rows to prevent layout shift.
• Scoped to branch for further UI issue investigations and resolution before merging.

Quality & Integrity

• Zero version change or release.
• All tests passing locally.