Remove non-standard CSS selectors and lock design-token correctness

[VatsalSy]

Why

Frontend reviews flagged invalid selectors and token mismatches that are easy to reintroduce and hard to spot visually.

Evidence

• PR JavaScript optimization: eliminate redundancy and improve performance #58 review: :contains() and :has(...:contains(...)) are non-standard and fail static analysis.
• PR JavaScript optimization: eliminate redundancy and improve performance #58 review: undefined/misspelled token names in research CSS.
• PR Harden content and script maintenance safeguards #62 migration work: tag selector moved toward .tags, indicating prior selector fragility.

Scope

• Replace non-standard selectors with stable class-based hooks in markup/CSS.
• Standardize tag selectors to class-based patterns (.tags ...) across affected styles/scripts.
• Fix token-name drift and undefined CSS variable references.
• Add lint enforcement for unknown selectors/undefined custom properties where possible.

Acceptance Criteria

• No CSS file uses :contains() or text-matching selector patterns.
• Tag styling is fully class-based and consistent with markup.
• CSS token references resolve cleanly (no undefined var warnings).
• Style/lint checks pass without suppressing these classes of errors.