feat(billing): add 'Manage subscription' link in meterDrawer footer #4056

Context

The compute mini-bar opens a drawer (billing.meterDrawer.component.vue) that shows :

• usage progress
• breakdown chart
• "Buy extras" button (opens checkout modal)

But there is no link to navigate to the full /billing page to manage subscription, change plan, view invoices, etc.

Today's only nav surfaces toward /billing are :

1. <organization-detail> page → "Manage subscription" button (visible only to org owners/admins, gated on serverConfig.billing.* — currently broken upstream, see pierreb-devkit/Node#XXX)
2. Direct URL typed manually

So a user who clicks the mini-bar gauge sees the drawer but has no obvious path to the broader billing surface — they end up using the back button or typing the URL.

Proposal

Add a footer with a "Manage subscription" link button in the drawer, after the breakdown + extras CTA. Clicking it :

• Closes the drawer (emit('update:modelValue', false))
• Routes to /billing

<!-- billing.meterDrawer.component.vue, before </v-card> closing tag -->
<v-divider class="my-4" />
<v-card-actions class="px-6 pb-6">
  <v-btn
    variant="text"
    color="primary"
    :class="config.vuetify.theme.rounded"
    class="text-none"
    to="/billing"
    @click="$emit('update:modelValue', false)"
  >
    <v-icon icon="fa-solid fa-credit-card" size="small" class="mr-2" />
    {{ t('billing.usage.manageSubscription') }}
  </v-btn>
</v-card-actions>

Plus the i18n key in src/modules/billing/lang/{en,fr}.js :

  billing: {
    usage: {
      title: 'Usage',
+     manageSubscription: 'Manage subscription',
      ...

Rationale

• Discoverability : the gauge is the most visible billing affordance for users on a meterMode plan. Not having a path from gauge → billing settings forces a non-obvious detour through the org page.
• Consistency : the <organization-detail> page already has "Manage subscription" button. The drawer should mirror that affordance.
• Low risk : single component edit + i18n + 1 unit test that the link exists.

Out of scope

• Adding billing to main nav / sidebar : explicitly NOT desired (per architectural decision — billing belongs to org context, not user nav).
• Adding billing to user profile : also out of scope (billing is org-level).

Effort

~10 min implementation + i18n + unit test, ~30 min PR convergence cycle.

Related

• pierreb-devkit/Node issue : exposing billing.{enabled,meterMode} in /api/auth/config (companion fix — without that, the drawer never appears in the first place because the mini-bar is hidden)
• comes-io/trawl_vue PR-T-V2 #819 — the original implementation that landed the drawer