Add shadcn/ui design system, theme, and base components

Install shadcn/ui with base-nova style, tailwind-merge, tw-animate-css,
and font packages. Add oklch-based light/dark theme in index.css and
17 base UI components (button, card, dialog, table, tabs, etc.) plus
shared lib utilities (format, colors, cn) and useSettings hook.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/components.json

@@ -0,0 +1,25 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "base-nova",
+  "rsc": false,
+  "tsx": true,
+  "tailwind": {
+    "config": "",
+    "css": "src/index.css",
+    "baseColor": "zinc",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "iconLibrary": "lucide",
+  "rtl": false,
+  "aliases": {
+    "components": "@/components",
+    "utils": "@/lib/utils",
+    "ui": "@/components/ui",
+    "lib": "@/lib",
+    "hooks": "@/hooks"
+  },
+  "menuColor": "default-translucent",
+  "menuAccent": "subtle",
+  "registries": {}
+}