fix: Improve editor selection and project tag contrast

[kof]

Closes #6301

Summary

• Restore Dashboard project tags to the pre-color-refactor treatment: fixed white text over a 30% black scrim in both application themes.
• Use one high-contrast text-selection treatment across native inputs, textareas, contenteditable fields, and every shared CodeMirror editor.
• Preserve the existing subtle selection treatment for Navigator, Project Settings, command menus, Dashboard navigation, and other selected controls.
• Add direct Storybook coverage for the CSS fragment editor.
• Simplify chromatic color derivation from 571 to 329 lines without changing computed colors.

Technical choices

• Keep project tags independent from application-theme polarity because they overlay arbitrary thumbnail artwork.
• Keep the existing selectionBackground control-state export unchanged at its original 12% light / 20% dark accent blend.
• Define separate --background-text-selection and --foreground-on-text-selection tokens exclusively for selected text.
• Use the bounded accent in light mode and a luminance-lifted accent in dark mode so selected text remains distinct even with extreme custom theme colors.
• Style CodeMirror through its existing selection layer and ::selection CSS hooks; no selection tracking or editor-state logic is added.
• Leave canvas rich-text selection independent: authored canvas documents do not load the Builder UI theme stylesheet.
• Replace five duplicated XYZ clamp formulas with the exact upper-luminance cap they reduce to. The removed lower-bound and white-mixing branches were unreachable because the minimum was fixed at zero.

Todo

• Restore the original project-tag scrim and foreground in both themes.
• Inventory native, CodeMirror, and canvas rich-text selection paths.
• Unify native and shared CodeMirror text-selection colors.
• Keep text selection independent from selected control and navigation states.
• Add the missing CSS fragment editor story.
• Add light- and dark-mode browser regressions and confirm they fail before the fix.
• Enforce 4.5:1 selected-text contrast and 3:1 highlight-to-surface contrast across supported theme inputs.
• Remove duplicated and unreachable chromatic color derivation while preserving outputs.
• Review documentation impact; the design-system color-token reference is updated, with no user-facing product documentation change required.
• Verify fixture impact; no fixture inputs or generated outputs are affected.

Verification

• pnpm --filter @webstudio-is/design-system test — 227 tests passed, including the control/text separation regression, Navigator tree interaction, default-palette comparisons, and the full supported-theme contrast sweep
• pnpm --filter @webstudio-is/design-system typecheck
• pnpm --filter @webstudio-is/builder exec vitest run app/shared/code-editor-base.browser.test.tsx --project app-browser-shared
• pnpm --filter @webstudio-is/builder exec vitest run app/dashboard/projects/project-card.browser.test.tsx --project app-browser-canvas-dashboard
• pnpm --filter @webstudio-is/builder typecheck
• Targeted oxlint, formatting, and git diff --check
• Live dark-mode Dashboard verification of the restored subtle navigation selection
• Storybook visual checks for the full color system and input/CodeMirror selection in light and dark modes