fix: resolve frontend code quality issues

- Create missing tsconfig.node.json for Vite config compilation
- Create postcss.config.js for Tailwind CSS processing
- Add test dependencies (vitest, testing-library, jsdom)
- Remove missing /vite.svg favicon reference from index.html

openspec/config.yaml

@@ -0,0 +1,20 @@
+schema: spec-driven
+
+# Project context (optional)
+# This is shown to AI when creating artifacts.
+# Add your tech stack, conventions, style guides, domain knowledge, etc.
+# Example:
+#   context: |
+#     Tech stack: TypeScript, React, Node.js
+#     We use conventional commits
+#     Domain: e-commerce platform
+
+# Per-artifact rules (optional)
+# Add custom rules for specific artifacts.
+# Example:
+#   rules:
+#     proposal:
+#       - Keep proposals under 500 words
+#       - Always include a "Non-goals" section
+#     tasks:
+#       - Break tasks into chunks of max 2 hours