feat(admin): configurable tabs via admin config array

[PierreBrisorgueil]

Problem

The admin page (/admin) has hardcoded tabs (Users, Organizations). Downstream projects need to add their own admin tabs (e.g. Costs, Logs, Billing) but currently have to either:

• Fork admin.view.vue and hardcode new tabs
• Create separate sidenav items (bad UX — splits admin into multiple nav entries)

Proposed solution

Add an adminTabs array to the admin module config (or the app-level config) that downstream projects can extend:

```js
// config/index.js (downstream project)
admin: {
tabs: [
{ value: 'costs', label: 'Costs', icon: 'fa-solid fa-coins', route: '/admin/costs' },
],
}
```

admin.view.vue reads this config and renders extra tabs alongside the default Users/Organizations tabs.

Expected behavior

• Default tabs (Users, Organizations) always rendered
• Extra tabs from config rendered after defaults
• Each extra tab is a <router-link> / <v-tab :to="tab.route"> pointing to the downstream route
• Tab hidden if user doesn't have manage UserAdmin ability (same guard as today)
• No change to existing downstream projects that don't use this config

Acceptance criteria

• config.admin.tabs array read in admin.view.vue
• Each entry: { value, label, icon, route } — icon optional
• Tabs rendered as v-tab with <router-link> (or to prop)
• Falls back gracefully to current behavior when array is empty/undefined
• Unit test: renders extra tab when config provides one
• Downstream usage example in devkit docs or comment

Context

First downstream use case: trawl_vue needs a "Costs" tab → /admin/costs (cost tracking per history — D5.5).