Extract snatch selection into reusable composable (#30)

## Summary

  - add a useSnatchSelection composable that centralises snatch shortfall calculations, position filtering, and RPC memoisation
  - refactor StakeHolder.vue to consume the composable instead of reimplementing the flow inline
  - introduce Vitest config and first composable tests (useSnatchSelection.spec.ts) to cover empty/partial fills and ownership edge cases
  - wire up project tooling updates so the new tests run (jsdom dep, updated package metadata)

  ## Testing

  - cd web-app && npm install
  - npm test

resolves #24

Co-authored-by: openhands <openhands@all-hands.dev>
Reviewed-on: https://codeberg.org/johba/harb/pulls/30

web-app/vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from 'vitest/config'
+import vue from '@vitejs/plugin-vue'
+import { fileURLToPath } from 'node:url'
+
+export default defineConfig({
+  plugins: [vue()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+  },
+  resolve: {
+    alias: {
+      '@': fileURLToPath(new URL('./src', import.meta.url))
+    }
+  }
+})