Move groupware account switcher and store into shared package

[kulmann]

Initial Checklist

• I understand this is a user story and questions should be posted in the Community Discussions
• I searched issues and couldn’t find anything (or linked relevant results below)

User Story

• As a developer, I want to re-use the account switcher from the mail app for another app so that I don't have to copy all the code.

User Value

DX / DRY

Acceptance Criteria

• move shared code to web-pkg:
  • accounts pinia store -> web-pkg/src/composables/piniaStores/groupware/groupwareAccounts.ts
  • account fetching logic -> web-pkg/src/composables/groupware/useLoadAccounts.ts
  • account switcher -> web-pkg/src/components/Groupware/Accounts/MailAccountsSwitch.vue
• each groupware app (mail / contacts / in the future also calendar) triggers fetching accounts on first visit if necessary
• must be possible to run a callback after an account gets selected (maybe just by using a watcher on the selected account in the pinia store), so that e.g. the Mail App can select the active mailbox and fetch emails after the account has been switched.