feat(admin): surface API errors in admin store actions

[PierreBrisorgueil]

Problem

All admin store actions (getUsers, getOrganizations, getCosts, etc.) swallow errors with console.log(err) and no user feedback. When an admin API call fails (network error, 403, 500), the view silently shows empty data with no indication of failure.

Proposed fix

Add an error state to the admin store and set it in each catch block. Surface it in admin.view.vue with a v-alert or snackbar.

// store
state: { ..., error: null },
// in each catch:
this.error = err?.response?.data?.message || 'Failed to load data';

Downstream

This pattern is used in all downstream admin views (trawl, comes, montaine, pierreb). Fix in the stack flows down via /update-stack.