feat(ui): add shadcn/ui infrastructure and migrate all buttons

Phase 0: Set up shadcn/ui foundation with OKLCH theme variables,
ThemeProvider, useThemeStore, cn() utility, and components.json.

Phase 1: Replace all raw <button> elements across 9 files (~25 buttons)
with the shadcn Button component wrapper, preserving functional colors
(indigo, blue, emerald tints) via className overrides where needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "https://ui.shadcn.com/schema.json",
+  "style": "new-york",
+  "rsc": false,
+  "tsx": true,
+  "tailwind": {
+    "config": "",
+    "css": "src/renderer/src/assets/main.css",
+    "baseColor": "neutral",
+    "cssVariables": true,
+    "prefix": ""
+  },
+  "aliases": {
+    "components": "@renderer/components",
+    "utils": "@renderer/lib/utils",
+    "ui": "@renderer/components/ui",
+    "lib": "@renderer/lib",
+    "hooks": "@renderer/hooks"
+  },
+  "iconLibrary": "lucide"
+}