web: component tests for the Share dialog

@vue/test-utils and jsdom as dev dependencies, jsdom chosen per file with the
vitest environment comment so the existing unit tests stay on node. The dialog's
repo.shares is faked; the tests cover offering everyone, sharing with a person
and a group, changing and removing a share (and the board's shared flag that
follows), a refused share keeping the choice, the load retry, and the
single-person instance. #5313.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/package.json

@@ -16,7 +16,9 @@
   },
   "devDependencies": {
     "@vitejs/plugin-vue": "^5.1.0",
+    "@vue/test-utils": "^2.5.1",
     "autoprefixer": "^10.4.0",
+    "jsdom": "^30.1.2",
     "postcss": "^8.4.0",
     "tailwindcss": "^3.4.0",
     "typescript": "~5.5.0",