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/package.json

@@ -9,7 +9,8 @@
     "preview": "vite preview",
     "build-only": "vite build",
     "type-check": "vue-tsc --build",
-    "subtree": "git subtree push --prefix dist origin gh-pages"
+    "subtree": "git subtree push --prefix dist origin gh-pages",
+    "test": "vitest"
   },
   "dependencies": {
     "@tanstack/vue-query": "^5.64.2",
@@ -35,6 +36,7 @@
     "@vitejs/plugin-vue": "^5.2.1",
     "@vue/tsconfig": "^0.7.0",
     "gh-pages": "^6.1.1",
+    "jsdom": "^27.0.0",
     "npm-run-all2": "^7.0.2",
     "typescript": "~5.7.3",
     "vite": "^6.0.11",