docs: add design system reference for frontend UI

DESIGN.md

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+# Polaris Task Force Design System
+
+## 1. Atmosphere & Identity
+
+Polaris is a dark operations console: focused, quiet, and information-dense without feeling cramped. The signature is a neutral charcoal surface hierarchy with restrained white controls and small, purposeful state accents.
+
+## 2. Color
+
+### Palette
+
+| Role | Token | Usage |
+|------|-------|-------|
+| Surface / page | `bg-background` | Frontend page background |
+| Surface / card | `bg-card` | Elevated content sections |
+| Text / primary | `text-foreground` | Headings and primary labels |
+| Text / secondary | `text-muted-foreground` | Descriptions and supporting copy |
+| Interactive / primary | `bg-primary`, `text-primary-foreground` | Primary actions and checked controls |
+| Interactive / focus | `ring-ring` | Keyboard focus indicators |
+| Border | `border-border` | Card and control boundaries |
+| Status / error | `text-destructive` | Inline validation and failure copy |
+| Status / success | `text-emerald-500` | Existing save confirmations |
+
+### Rules
+
+- Use semantic Tailwind tokens backed by `src/app/(frontend)/styles.css`; do not add raw colors in frontend components.
+- Preserve the dark neutral palette and use accent color only for interactive state.
+
+## 3. Typography
+
+### Scale
+
+| Level | Token | Usage |
+|-------|-------|-------|
+| Page title | `text-lg font-semibold` | Route heading |
+| Card title | `font-semibold` | Section heading |
+| Body | inherited `text-base` | Primary content |
+| Supporting | `text-sm text-muted-foreground` | Descriptions and helper text |
+| Compact | `text-xs` | Secondary metadata only |
+
+### Font Stack
+
+- Primary: Geist, sans-serif
+- Mono: Geist Mono, monospace
+
+## 4. Spacing & Layout
+
+### Base Unit
+
+Spacing follows Tailwind's 4px scale. Existing frontend stacks use `gap-1`, `gap-2`, `gap-3`, `gap-4`, and `gap-6`.
+
+### Rules
+
+- Use `flex flex-col gap-*` for vertical stacks; do not use `space-y-*`.
+- Account content is constrained with `max-w-2xl` and uses `gap-6` between cards.
+- Preference sections use `CardHeader` and `CardContent`; controls use `gap-4`.
+- Preference sub-options are indented with `pl-6` and remain one readable column on narrow screens.
+
+## 5. Components
+
+### Card sections
+
+- **Structure**: `Card` → `CardHeader` → `CardTitle` + `CardDescription`; `CardContent` body.
+- **Variants**: default dark card surface.
+- **Spacing**: built-in card padding; internal control stack uses `gap-4`.
+- **States**: default, focus/error states belong to child controls.
+- **Accessibility**: headings and descriptions provide section context.
+- **Motion**: no section-level motion.
+- **Layout**: vertical stack.
+
+### Preference row
+
+- **Structure**: `label.flex.items-center.justify-between` with text on the left and a shadcn control on the right.
+- **Variants**: primary setting and indented notification setting.
+- **Spacing**: `gap-3` for label copy; `pl-6` for dependent settings.
+- **States**: default, checked, unchecked, disabled, focus-visible.
+- **Accessibility**: the label owns the control relationship; disabled dependent rows use both disabled controls and reduced-opacity presentation.
+- **Motion**: shadcn control transition only.
+- **Layout**: responsive cluster with a single-column text block.
+
+### Switch
+
+- **Structure**: shadcn `Switch` with an associated label and optional description.
+- **Variants**: primary and dependent/disabled.
+- **Spacing**: control row uses `gap-4`.
+- **States**: unchecked, checked, hover, active, focus-visible, disabled.
+- **Accessibility**: keyboard reachable with visible focus and native switch semantics.
+- **Motion**: 100–150ms control transition.
+- **Layout**: inline control at the row edge.
+
+### RadioGroup
+
+- **Structure**: shadcn `RadioGroup` with `RadioGroupItem` and `Label` pairs.
+- **Variants**: two-option roster label selection.
+- **Spacing**: option cluster uses `gap-4`.
+- **States**: selected, unselected, hover, active, focus-visible.
+- **Accessibility**: grouped options retain native radio semantics and labels.
+- **Motion**: control transition only.
+- **Layout**: wraps on narrow screens without horizontal overflow.
+
+## 6. Motion & Interaction
+
+### Timing
+
+| Type | Duration | Usage |
+|------|----------|-------|
+| Micro | 100–150ms | Switch and radio state feedback |
+| Standard | 200–300ms | Button hover and focus transitions |
+
+### Rules
+
+- Keep motion in existing shadcn controls and buttons; do not add decorative animation to the account form.
+- Preserve visible keyboard focus and `prefers-reduced-motion` behavior from the component primitives.
+
+## 7. Depth & Surface
+
+The frontend uses a mixed strategy: card borders plus the existing shadcn card shadow, with tonal changes for muted controls. New preference sections should compose the existing `Card` primitive rather than introduce custom shadows or radii.
+
+## 8. Accessibility Constraints & Accepted Debt
+
+### Constraints
+
+- Target WCAG 2.2 AA.
+- Every interactive control must be keyboard reachable with a visible focus state.
+- Dependent notification controls must expose their disabled state through the actual control, not only visual opacity.
+- The form must reflow to one readable column at mobile widths without horizontal scrolling.
+
+### Accepted Debt
+
+| Item | Location | Why accepted | Owner / Exit |
+|------|----------|--------------|--------------|
+| Existing success color uses `text-emerald-500` | Account forms | Preserve established feedback styling in this focused redesign | Consolidate account feedback tokens in a future account UI pass |