refactor(web-ui): govern surface theme token namespaces

[limityan]

Summary

• Migrates remaining Flow Chat/tool-card surface-local CSS variable names to explicit namespaces, including BaseToolCard, SnapshotCard, ScrollAnchor, and failed user-message sizing keys.
• Consolidates exact command-empty color usage through --tool-command-empty-rgb, and moves widget/editor highlight defaults into shared token definitions without changing their RGB values.
• Adds SURFACE_TOKEN_RENAME_CONTRACTS plus audit/baseline enforcement so deprecated local keys cannot return silently and rename canonical targets must continue to exist.
• Updates the theme optimization and visual governance docs to reflect the completed 6-9 surface coverage and the tightened terminal/domain metrics.

Risk Review

• No intentional user-visible color change: renamed or moved values keep the same colors and alpha behavior.
• No adjacent status-color merge: git/diff accepted/rejected/conflict semantics, terminal ANSI palette, Monaco theme palette, Mermaid, Prism, and generated widget payload compatibility are left intact.
• Dynamic CSS key review: target deprecated keys are not generated through runtime string composition; existing dynamic families remain the registered theme families audited by theme:color-audit.
• Generated widget payload remains unchanged at 326 vars; the new host UI tokens are not exported into the iframe payload.
• Rename contract drift is now guarded in both directions: old keys must stay absent, and each registered canonical target must remain defined.

Validation

• pnpm run theme:color-audit:test
• pnpm run theme:color-audit
• pnpm run theme:visual-contract
• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run
• pnpm run build:web
• git diff --check

pnpm run build:web completed with existing Vite chunk/dynamic-import warnings only.