feat(admin/plugins): install/update/uninstall UI + verbatim capability review — closes #253

[tayebmokni]

Summary

• Builds the admin-side plugin management screens at apps/admin/src/app/plugins/: a list view, per-plugin detail view, and the install flow centred on a verbatim capability review that lists every requested permission in plain English with explicit operator consent before the install fires.
• Adds server actions wrapping /api/v1/plugins/{install,activate,deactivate,:name} with friendly "endpoint not yet deployed" fallbacks for the in-flight REST work (issues fix(ci): vet/test workspace modules only, not transitive deps #340 / feat(auth): login flow + 2FA — closes #124 #341).
• Mirrors packages/go/plugins/capabilities/registry.go::builtinCapabilityDefs into a TS lookup so the consent UI renders meaningful descriptions ("Send transactional email on behalf of this site") before the host receives the install request. Sensitive caps are visually flagged; unknown ids surface as "Unrecognised" so out-of-band manifests can’t be silently consented to.

File layout

apps/admin/src/app/plugins/
  page.tsx                          RSC list view, defensive fetch
  PluginListClient.tsx              search / filter / row actions / uninstall modal
  actions.ts                        server actions over /api/v1/plugins/*
  types.ts                          TS projection of lifecycle.Plugin
  capability-registry.ts            TS mirror of host cap registry
  [name]/
    page.tsx                        RSC detail fetch
    PluginDetailView.tsx            manifest summary + caps + deps + last-error
  install/
    page.tsx                        install entry
    InstallForm.tsx                 upload / paste flow + capability review
  components/
    CapabilityReview.tsx            the verbatim consent screen
    PluginStatusBadge.tsx           shared lifecycle-state pill

Sidebar gains a /plugins link alongside Users and Settings.

Verification

• pnpm install --frozen-lockfile=false — clean
• pnpm --filter @gonext/admin test — 87 / 87 passing (33 new tests across 6 new test files)
• pnpm --filter @gonext/admin typecheck — clean
• pnpm --filter @gonext/admin lint — no errors

Test plan

• List page renders one row per plugin from the fetched data
• Empty / filter-empty / fetch-error states render without crashing
• Detail view renders capabilities as human-readable text in declaration order
• Dependency status renders satisfied / unsatisfied chips, propagates reason
• errored plugins surface the last-error block as a role="alert"
• Install page: file upload (manifest.json) triggers the manifest preview
• Install page: pasting JSON triggers the manifest preview
• Capability review: every cap from the manifest appears in declaration order
• Capability review: sensitive caps get the "Sensitive" tag; unknown ids get "Unrecognised"
• Capability review: duplicate caps render twice (verbatim, no dedup)
• Install button is disabled until the consent checkbox is ticked
• Uninstall opens a confirmation modal; Cancel closes without firing the action
• Uninstall confirm calls the server action with the right slug
• Active plugins disable the Uninstall button until deactivated (with title= hint)
• Errored plugins can be uninstalled (after confirmation)
• Sidebar test updated for the new /plugins entry

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