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.

lib/plugin-sandbox/loader.ts

@@ -41,11 +41,16 @@ export function setSandboxStoreAccessor(a: StoreAccessor): void { storeAccessor
 
 let currentLocale = 'en';
 export function setSandboxLocale(locale: string): void {
+  // Ignore empty/falsy values so a not-yet-seeded locale store can't clobber a
+  // good locale back to '' - the initial 'en' default stands until the real
+  // locale arrives via the store subscription.
+  if (!locale) return;
   currentLocale = locale;
-  // Push to all active background instances.
-  // Slot iframes inherit locale at spawn time; they're short-lived.
-  // (We don't import the registry here to avoid a circular import; the
-  //  PluginIframeSlot subscribes to locale changes on its own.)
+  // Background instances read `currentLocale` at load time; the slot-iframe
+  // component reads this global at spawn time (plugin-iframe-slot.tsx). Keep
+  // both in step from one place. Already-running instances are not re-pushed,
+  // so a locale switch only affects plugins/slots loaded afterwards.
+  (globalThis as unknown as { __APP_LOCALE__?: string }).__APP_LOCALE__ = locale;
 }
 
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