Feature: localizable sandboxed plugins (manifest locales + api.i18n.t)

The plugin runtime received the active locale (init payload + 'locale-change')
and plugins could declare a `locales` map, but none of it was usable: the
locales never reached the runtime, and buildPluginApi exposed no i18n. So
plugin code calling pluginApi.i18n.t(...) (as the External Link Warning plugin
does) always got undefined and fell back to English.

Thread plugin locales end to end and surface an i18n API:
- ServerPlugin gains `locales`; the upload route persists manifest.locales
  (alongside configSchema/settingsSchema), and /api/plugins surfaces it to the
  client so it flows registry -> client -> sandbox host-bridge -> runtime.
- runtime sets __PLUGIN_LOCALE__ at init (not only on later 'locale-change')
  and buildPluginApi exposes `i18n.locale` + `i18n.t(key, vars)` resolving
  against the plugin's declared locales (manifest.locales) with English/key
  fallback and {placeholder} interpolation.

Lets any sandboxed plugin localize its strings from its manifest.

app/api/admin/plugins/route.ts

@@ -192,6 +192,9 @@ export async function POST(request: NextRequest) {
       ...(manifest.settingsSchema && typeof manifest.settingsSchema === 'object'
         ? { settingsSchema: manifest.settingsSchema as ServerPlugin['settingsSchema'] }
         : {}),
+      ...(manifest.locales && typeof manifest.locales === 'object'
+        ? { locales: manifest.locales as ServerPlugin['locales'] }
+        : {}),
       ...(declaredFrameOrigins.length > 0
         ? { frameOrigins: declaredFrameOrigins }
         : {}),

app/api/plugins/route.ts

@@ -57,6 +57,9 @@ export async function GET() {
       // Per-user settings schema, captured from the manifest at upload/load
       // time so the client can render the settings UI without re-parsing.
       settingsSchema: p.settingsSchema,
+      // Plugin-declared i18n tables, so the sandbox can localize plugin
+      // strings via api.i18n.t().
+      locales: p.locales,
     }));
 
     // Only serve enabled themes