refactor(form): use semantic tokens and fix dark control backgrounds

[joshsadam]

Problem

Form controls still use raw neutral-*, primary-*, and hard-coded black/white focus outlines.

The highest-priority violation is in checkbox/radio shared styles: controls use dark:bg-neutral-950, but the design docs reserve neutral-950 for dense inset regions like the data grid body, not everyday form controls.

Switch styles also drift from the new system. They use raw neutral/primary classes and black/white peer focus outlines.

What we will build

Form controls that use semantic colour tokens, correct dark-mode surfaces, and the shared focus ring.

How we will build it

• Update app/components/pathogen/form/form_styles.rb.
• Update app/components/pathogen/form/switch_styles.rb.
• Confirm radio_button_styles.rb continues to inherit the corrected shared control styles.
• Replace label/help text colours with --pvc-color-text and --pvc-color-text-muted where equivalent.
• Replace control borders/surfaces with --pvc-color-border, --pvc-color-border-strong, --pvc-color-surface, and --pvc-color-surface-muted where appropriate.
• Replace dark:bg-neutral-950 on everyday controls with a regular surface token.
• Replace focus-visible:outline-black / outline-white and peer equivalents with outline-[var(--pvc-color-focus)].
• Map checked/accent states to --pvc-color-accent / --pvc-color-accent-solid as appropriate.
• Keep native HTML controls and public APIs unchanged.
• Touch form previews only where needed to demonstrate corrected component states. Broader Lookbook aesthetic cleanup is tracked separately.

Testing

• Checkbox label, help text, checked, disabled, hover, and focus states in light and dark mode.
• Radio label, help text, checked, disabled, hover, and focus states in light and dark mode.
• Switch label, state text, checked, disabled, hover, and focus states in light and dark mode.
• Dark controls no longer use neutral-950 backgrounds.
• Focus is visible and distinct on keyboard tab through sample form previews.
• Relevant form component tests pass.
• pnpm run build:css:check passes if class generation changes the CSS artifact.

Not in scope

• New form components or builder API changes.
• Broad preview restyling beyond the component states needed here.
• Host-app form migrations.

Done when

Form controls match the Foundations colour, focus, and dark-mode rules, and the form-related dark-mode checklist items pass.

Depends on: #88
Design docs: docs/lookbook/design_system/01-design-contract.md.erb (Forms, Focus And Keyboard); docs/lookbook/design_system/03-dark-mode.md.erb (The Five Qualities In Dark Mode, Check Before Merge)