feat: add web frontend rules and design quality hook

rules/web/coding-style.md

@@ -0,0 +1,96 @@
+> This file extends [common/coding-style.md](../common/coding-style.md) with web-specific frontend content.
+
+# Web Coding Style
+
+## File Organization
+
+Organize by feature or surface area, not by file type:
+
+```text
+src/
+├── components/
+│   ├── hero/
+│   │   ├── Hero.tsx
+│   │   ├── HeroVisual.tsx
+│   │   └── hero.css
+│   ├── scrolly-section/
+│   │   ├── ScrollySection.tsx
+│   │   ├── StickyVisual.tsx
+│   │   └── scrolly.css
+│   └── ui/
+│       ├── Button.tsx
+│       ├── SurfaceCard.tsx
+│       └── AnimatedText.tsx
+├── hooks/
+│   ├── useReducedMotion.ts
+│   └── useScrollProgress.ts
+├── lib/
+│   ├── animation.ts
+│   └── color.ts
+└── styles/
+    ├── tokens.css
+    ├── typography.css
+    └── global.css
+```
+
+## CSS Custom Properties
+
+Define design tokens as variables. Do not hardcode palette, typography, or spacing repeatedly:
+
+```css
+:root {
+  --color-surface: oklch(98% 0 0);
+  --color-text: oklch(18% 0 0);
+  --color-accent: oklch(68% 0.21 250);
+
+  --text-base: clamp(1rem, 0.92rem + 0.4vw, 1.125rem);
+  --text-hero: clamp(3rem, 1rem + 7vw, 8rem);
+
+  --space-section: clamp(4rem, 3rem + 5vw, 10rem);
+
+  --duration-fast: 150ms;
+  --duration-normal: 300ms;
+  --ease-out-expo: cubic-bezier(0.16, 1, 0.3, 1);
+}
+```
+
+## Animation-Only Properties
+
+Prefer compositor-friendly motion:
+- `transform`
+- `opacity`
+- `clip-path`
+- `filter` (sparingly)
+
+Avoid animating layout-bound properties:
+- `width`
+- `height`
+- `top`
+- `left`
+- `margin`
+- `padding`
+- `border`
+- `font-size`
+
+## Semantic HTML First
+
+```html
+<header>
+  <nav aria-label="Main navigation">...</nav>
+</header>
+<main>
+  <section aria-labelledby="hero-heading">
+    <h1 id="hero-heading">...</h1>
+  </section>
+</main>
+<footer>...</footer>
+```
+
+Do not reach for generic wrapper `div` stacks when a semantic element exists.
+
+## Naming
+
+- Components: PascalCase (`ScrollySection`, `SurfaceCard`)
+- Hooks: `use` prefix (`useReducedMotion`)
+- CSS classes: kebab-case or utility classes
+- Animation timelines: camelCase with intent (`heroRevealTl`)