Let plugins localize their admin page labels (sidebar + command palette)

[swissky]

Problem

The admin UI is fully localized via Lingui, but plugin adminPages labels are rendered verbatim from the manifest. A plugin can localize every string inside its pages (by merging a Lingui catalog into the shared i18n instance from @lingui/core — the prebuilt admin keeps Lingui external, so it's the same singleton), yet its sidebar entry and command-palette item stay hard-coded English. In a German admin you get "Bestellungen" everywhere inside the page but "Orders" in the nav next to it.

There is currently no way for a plugin to fix this: the label is a plain string in the manifest, and the sidebar/palette never consult the i18n instance.

Proposal

Run declared plugin page labels through the shared Lingui instance at render time — i18n._(page.label) — in the two places that render them (Sidebar.tsx, AdminCommandPalette.tsx).

• Plugins that load a catalog (English label as msgid) get localized navigation, consistent with how they already localize their page content.
• Labels matching one of the admin's own messages (e.g. "Settings") follow the admin locale automatically — a localized admin no longer shows stray English nav items.
• Labels with no catalog entry anywhere render unchanged (i18n._() falls back to the msgid), so existing plugins are unaffected.
• The prettified-plugin-id fallback (no label declared) is not translated — it's derived from the package name, not authored English.

No manifest or API changes; purely a render-time lookup. Alternative considered: per-locale label maps in the manifest (label: { en: "...", de: "..." }) — rejected as a breaking/parallel mechanism when the admin already has a i18n pipeline plugins can feed.

I have a working branch with the change, a shared resolvePluginPageLabel() helper (deduplicates the sidebar/palette copy-paste), tests, and docs — happy to open the PR if this direction sounds right.