feat: add duckdb-wasm singleton for client-side parquet queries

Adds src/utils/duckdbClient.js, a lazy-initialized getDb() singleton
wrapping the AsyncDuckDB MVP (single-threaded) wasm bundle. Pins
@duckdb/duckdb-wasm to ^1.32.0 (npm's latest dist-tag currently points
at a -dev prerelease). Excludes the package from Vite's dev-server
dependency pre-bundling so its worker/wasm ?url imports resolve
correctly.

Verified live in the browser under the /crdc-demo/ base path: wasm and
worker assets load with 200, and a SELECT 42 query round-trips
correctly through the singleton, confirming the risk flagged in the
design spec (duckdb-wasm loading correctly from a subpath-served Vite
dev server) does not materialize.

package.json

@@ -18,6 +18,7 @@
   "author": "Civilytics Consulting LLC",
   "license": "MIT",
   "devDependencies": {
+    "@duckdb/duckdb-wasm": "^1.32.0",
     "@vitejs/plugin-react-swc": "^4.3.3",
     "eslint": "^8.57.1",
     "prettier": "^3.9.6",