feat(frontend): add design tokens, dark mode hook, and i18n scaffold

Part of the Ombrora Convert redesign (Task 1/5).

frontend/src/index.css

@@ -1,111 +1,73 @@
+@import url('https://fonts.googleapis.com/css2?family=Plus+Jakarta+Sans:wght@400;500;600;700&display=swap');
+
 :root {
-  --text: #6b6375;
-  --text-h: #08060d;
-  --bg: #fff;
-  --border: #e5e4e7;
-  --code-bg: #f4f3ec;
-  --accent: #aa3bff;
-  --accent-bg: rgba(170, 59, 255, 0.1);
-  --accent-border: rgba(170, 59, 255, 0.5);
-  --social-bg: rgba(244, 243, 236, 0.5);
-  --shadow:
-    rgba(0, 0, 0, 0.1) 0 10px 15px -3px, rgba(0, 0, 0, 0.05) 0 4px 6px -2px;
+  --color-primary: #2563eb;
+  --color-on-primary: #ffffff;
+  --color-secondary: #3b82f6;
+  --color-accent: #d97706;
+  --color-background: #f8fafc;
+  --color-foreground: #0f172a;
+  --color-muted: #f1f5fd;
+  --color-border: #e4ecfc;
+  --color-destructive: #dc2626;
+  --color-ring: #2563eb;
 
-  --sans: system-ui, 'Segoe UI', Roboto, sans-serif;
-  --heading: system-ui, 'Segoe UI', Roboto, sans-serif;
-  --mono: ui-monospace, Consolas, monospace;
+  --font-sans: 'Plus Jakarta Sans', system-ui, 'Segoe UI', Roboto, sans-serif;
 
-  font: 18px/145% var(--sans);
-  letter-spacing: 0.18px;
   color-scheme: light dark;
-  color: var(--text);
-  background: var(--bg);
-  font-synthesis: none;
-  text-rendering: optimizeLegibility;
-  -webkit-font-smoothing: antialiased;
-  -moz-osx-font-smoothing: grayscale;
-
-  @media (max-width: 1024px) {
-    font-size: 16px;
-  }
+  font: 16px/1.5 var(--font-sans);
 }
 
-@media (prefers-color-scheme: dark) {
-  :root {
-    --text: #9ca3af;
-    --text-h: #f3f4f6;
-    --bg: #16171d;
-    --border: #2e303a;
-    --code-bg: #1f2028;
-    --accent: #c084fc;
-    --accent-bg: rgba(192, 132, 252, 0.15);
-    --accent-border: rgba(192, 132, 252, 0.5);
-    --social-bg: rgba(47, 48, 58, 0.5);
-    --shadow:
-      rgba(0, 0, 0, 0.4) 0 10px 15px -3px, rgba(0, 0, 0, 0.25) 0 4px 6px -2px;
-  }
+:root[data-theme='dark'] {
+  --color-primary: #3b82f6;
+  --color-on-primary: #0f172a;
+  --color-secondary: #60a5fa;
+  --color-accent: #f59e0b;
+  --color-background: #0f172a;
+  --color-foreground: #f1f5f9;
+  --color-muted: #1e293b;
+  --color-border: #334155;
+  --color-destructive: #f87171;
+  --color-ring: #3b82f6;
+}
 
-  #social .button-icon {
-    filter: invert(1) brightness(2);
-  }
+*, *::before, *::after {
+  box-sizing: border-box;
 }
 
 body {
   margin: 0;
+  background: var(--color-background);
+  color: var(--color-foreground);
+  transition: background-color 200ms ease, color 200ms ease;
 }
 
 #root {
-  width: 1126px;
-  max-width: 100%;
-  margin: 0 auto;
-  text-align: center;
-  border-inline: 1px solid var(--border);
   min-height: 100svh;
   display: flex;
   flex-direction: column;
-  box-sizing: border-box;
 }
 
-h1,
-h2 {
-  font-family: var(--heading);
-  font-weight: 500;
-  color: var(--text-h);
+a {
+  color: var(--color-primary);
 }
 
-h1 {
-  font-size: 56px;
-  letter-spacing: -1.68px;
-  margin: 32px 0;
-  @media (max-width: 1024px) {
-    font-size: 36px;
-    margin: 20px 0;
+button, input, select {
+  font-family: inherit;
+}
+
+button {
+  cursor: pointer;
+}
+
+:focus-visible {
+  outline: 2px solid var(--color-ring);
+  outline-offset: 2px;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  *, *::before, *::after {
+    transition: none !important;
+    animation: none !important;
   }
 }
-h2 {
-  font-size: 24px;
-  line-height: 118%;
-  letter-spacing: -0.24px;
-  margin: 0 0 8px;
-  @media (max-width: 1024px) {
-    font-size: 20px;
-  }
-}
-p {
-  margin: 0;
-}
-
-code,
-.counter {
-  font-family: var(--mono);
-  display: inline-flex;
-  border-radius: 4px;
-  color: var(--text-h);
-}
-
-code {
-  font-size: 15px;
-  line-height: 135%;
-  padding: 4px 8px;
-  background: var(--code-bg);
-}