refactor(css): systematize z-index with semantic tokens

[Armansiddiqui9]

Closes #12363

refactor: Systematize z-index with semantic tokens

Addresses the "token system" workstream of issue #12363 by:

• Renaming numeric primitives (level-1, level-14, etc.) to semantic tokens (behind, base, raised, sticky, fixed, dropdown, modal, overlay, toast)
• Removing unused level-6 and level-7 primitives per intentional gaps policy
• Documenting the semantic scale and stacking hierarchy inline
• Clarifying rules: token-only usage, isolation: isolate for layering components, Popover API preference for new overlays

Technical

All 9 semantic tokens map to existing primitive values (no visual regressions). The token file now serves as the single source of truth for z-index hierarchy. Stylelint already enforces token-only usage via scale-unlimited/declaration-strict-value rule.

Deferred to follow-up PRs (per issue checklist):

• Replace hardcoded z-index values (plot.js, SelectionManager.css)
• Remove !important from ui-dialog.css and legacy-jquery-ui.css
• Add isolation: isolate to 5 components (.list-follow-card, .list-card, .rating-form .stars, .mybooks-details, .editionCover)

Testing

• Verify no CSS regressions: npm test or visual QA on existing components
• Confirm Stylelint accepts all token usage and rejects raw z-index values

Screenshot

Stakeholders

@lokesh