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.

vite.config.mjs

@@ -8,4 +8,10 @@ export default defineConfig({
   server: { port: 5173 },
   build: { outDir: 'dist' },
   base: '/crdc-demo/',  // Required for subdirectory deployment on git-pages
+  optimizeDeps: {
+    // duckdb-wasm ships its own worker + wasm binaries resolved via `?url`
+    // imports; esbuild's dev-server pre-bundling can rewrite those import
+    // paths and break worker instantiation. Exclude it from pre-bundling.
+    exclude: ['@duckdb/duckdb-wasm'],
+  },
 })