feat(styles): add shadow + motion tokens, surface utilities, mode-switching color vars

Establish a real design system foundation by moving the project from
inline arbitrary-value classes to named tokens and reusable utilities.

Tokens added to @theme (auto-generate Tailwind utilities):
- Shadows: shadow-rest, shadow-stamp-{rest,hover,pressed,card},
  shadow-popover, shadow-floating-panel{,-dark}, shadow-overlay
- Motion: duration-{fast,normal,slow,slower};
  ease-{standard,out-soft,spring-overshoot}

Semantic mode-switching colors added to :root / .dark so utilities
auto-adapt without dark: variants:
- --color-border-subtle, --color-text-subtle, --color-skeleton

Utilities migrated to Tailwind v4's @utility directive with direct CSS
properties (previously @layer utilities with @apply chains, which
silently failed when chaining to other user-defined utilities):
- border-subtle, text-subtle, focus-ring
- surface-canvas, surface-card, surface-card-elevated, surface-popover,
  surface-floating
- flex-center, skeleton-fill, text-label-mono

Notes:
- text-secondary was renamed to text-subtle because --color-secondary is
  registered in @theme (a near-white shadcn surface token), which made
  Tailwind v4 auto-generate a colliding text-secondary utility that won
  over the user-defined one — every consumer effectively rendered as
  near-white text. The text-subtle name pairs cleanly with border-subtle
  and avoids any @theme collisions.
- Dead --space-* variable scale removed (was defined but never wired
  into @theme; Tailwind's default spacing scale is used everywhere).

src/app/styles/app.css

@@ -14,6 +14,12 @@
   --swiss-black: #1a1a1a;
   --swiss-white: #ffffff;
 
+  /* Semantic mode-switching colors. These are redefined inside `.dark`
+     so utilities that reference them auto-adapt without a `dark:` variant. */
+  --color-border-subtle: rgb(0 0 0 / 0.05);
+  --color-text-subtle: var(--neutral-500);
+  --color-skeleton: var(--neutral-200);
+
   /* Neutral Grays */
   --neutral-50: #fafafa;
   --neutral-100: #f5f5f5;
@@ -80,16 +86,6 @@
   --sidebar-border: oklch(0.922 0 0);
   --sidebar-ring: oklch(0.708 0 0);
 
-  /* Spacing Scale (rem-based) */
-  --space-xs: 0.25rem;
-  --space-sm: 0.5rem;
-  --space-md: 0.75rem;
-  --space-lg: 1rem;
-  --space-xl: 1.5rem;
-  --space-2xl: 2rem;
-  --space-3xl: 3rem;
-  --space-4xl: 4rem;
-
   /* Typography Scale */
   --text-xs: 0.75rem;
   --text-sm: 0.875rem;
@@ -114,6 +110,11 @@
   --color-surface: var(--dark-bg);
   --color-paper: var(--dark-card);
 
+  /* Dark-mode overrides for the semantic mode-switching colors. */
+  --color-border-subtle: rgb(255 255 255 / 0.1);
+  --color-text-subtle: var(--neutral-400);
+  --color-skeleton: var(--neutral-800);
+
   --background: oklch(0.145 0 0);
   --foreground: oklch(0.985 0 0);
   --card: oklch(0.145 0 0);
@@ -212,6 +213,51 @@
   --text-2xs: 0.625rem;
   /* Monospace label tracking — used in Loader and Footnote */
   --tracking-wider-mono: 0.2em;
+
+  /* ============================================
+     SHADOW TOKENS
+     ============================================ */
+
+  /* Default resting shadow — equivalent to Tailwind's shadow-sm. Used on
+     buttons, sliders, popover triggers in non-floating state. */
+  --shadow-rest: 0 1px 2px 0 rgb(0 0 0 / 0.05);
+
+  /* Swiss "hard offset" stamp — rests at 2px/2px, lifts to 3px/3px on
+     hover, presses back to 1px/1px on active. Primary button motif. */
+  --shadow-stamp-rest: 0.125rem 0.125rem 0 0 rgb(0 0 0 / 0.1);
+  --shadow-stamp-hover: 0.1875rem 0.1875rem 0 0 rgb(0 0 0 / 0.15);
+  --shadow-stamp-pressed: 0.0625rem 0.0625rem 0 0 rgb(0 0 0 / 0.1);
+
+  /* Card-tier hard-offset stamp — wider, brand-tinted. Used on
+     interactive cards (FontSampler hover). */
+  --shadow-stamp-card: 5px 5px 0 0 var(--color-brand);
+
+  /* Floating popovers (typography menu, combo control list). */
+  --shadow-popover: 0 20px 40px -10px rgb(0 0 0 / 0.15);
+
+  /* Drop-shadow under semi-translucent floating panels like the
+     comparison slider's character row. */
+  --shadow-floating-panel: 0 25px 50px -12px rgb(0 0 0 / 0.05);
+  --shadow-floating-panel-dark: 0 25px 50px -12px rgb(0 0 0 / 0.2);
+
+  /* Drawer / overlay shadow — full-strength shadow-2xl. */
+  --shadow-overlay: 0 25px 50px -12px rgb(0 0 0 / 0.25);
+
+  /* ============================================
+     MOTION TOKENS
+     ============================================ */
+
+  --duration-fast: 150ms;
+  --duration-normal: 200ms;
+  --duration-slow: 300ms;
+  --duration-slower: 500ms;
+
+  /* Tailwind's default ease-in-out — symmetric, good for layout shifts. */
+  --ease-standard: cubic-bezier(0.4, 0, 0.2, 1);
+  /* Decelerating curve — matches Tailwind's ease-out. Dominant in this codebase. */
+  --ease-out-soft: cubic-bezier(0, 0, 0.2, 1);
+  /* Spring overshoot — used in character pop animation. */
+  --ease-spring-overshoot: cubic-bezier(0.34, 1.56, 0.64, 1);
 }
 
 @layer base {
@@ -277,21 +323,86 @@
   }
 }
 
-@layer utilities {
+/* ============================================
-  /* 21× border-black/5 dark:border-white/10 → single token */
+   DESIGN-SYSTEM UTILITIES
-  .border-subtle {
+   ============================================
-    @apply border-black/5 dark:border-white/10;
+   Defined via `@utility` (Tailwind v4) so they integrate with the variant
-  }
+   system (`hover:`, `dark:`, breakpoints) and don't rely on `@apply`
-  /* Secondary text pair */
+   chains. Colors reference the mode-switching semantic vars defined in
-  .text-secondary {
+   `:root`/`.dark` above, so most utilities need no `dark:` variant in
-    @apply text-neutral-500 dark:text-neutral-400;
+   their definition or at call sites. */
-  }
+
-  /* Standard focus ring */
+@utility border-subtle {
-  .focus-ring {
+  border-color: var(--color-border-subtle);
-    @apply focus-visible:ring-2 focus-visible:ring-brand focus-visible:ring-offset-2;
+}
+
+/* Muted text color — paired with `border-subtle` naming. The previous
+   name `text-secondary` collided with Tailwind v4 auto-generating a
+   utility from `--color-secondary` (the shadcn near-white surface token
+   registered in `@theme`), which made every consumer effectively
+   invisible (near-white text on light backgrounds). */
+@utility text-subtle {
+  color: var(--color-text-subtle);
+}
+
+@utility focus-ring {
+  &:focus-visible {
+    outline: 2px solid transparent;
+    outline-offset: 2px;
+    box-shadow: 0 0 0 2px var(--color-background, white), 0 0 0 4px var(--color-brand);
   }
 }
 
+/* ── Surface utilities ────────────────────────────────────────── */
+
+@utility surface-canvas {
+  background-color: var(--color-surface);
+}
+
+@utility surface-card {
+  background-color: var(--color-paper);
+  border: 1px solid var(--color-border-subtle);
+}
+
+@utility surface-card-elevated {
+  background-color: var(--color-paper);
+  border: 1px solid var(--color-border-subtle);
+  box-shadow: var(--shadow-rest);
+}
+
+@utility surface-popover {
+  background-color: var(--color-paper);
+  border: 1px solid var(--color-border-subtle);
+  box-shadow: var(--shadow-popover);
+}
+
+@utility surface-floating {
+  background-color: color-mix(in srgb, var(--color-surface) 80%, transparent);
+  backdrop-filter: blur(12px);
+  border: 1px solid var(--color-border-subtle);
+}
+
+/* ── Shape / layout ───────────────────────────────────────────── */
+
+@utility flex-center {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+@utility skeleton-fill {
+  background-color: color-mix(in srgb, var(--color-skeleton) 70%, transparent);
+}
+
+/* ── Typography ───────────────────────────────────────────────── */
+
+@utility text-label-mono {
+  font-family: var(--font-primary);
+  font-weight: 700;
+  letter-spacing: -0.025em;
+  text-transform: uppercase;
+}
+
 /* Global utility - useful across your app */
 @media (prefers-reduced-motion: reduce) {
     * {