config: optional defaults.lang to pin a default UI language

When set (and the user hasn't explicitly chosen one), it overrides
browser detection after the runtime config loads. Omitted = auto-detect
as before.

public/sw.js

@@ -1,7 +1,7 @@
 // Tchatou service worker — installable PWA + offline app shell.
 // Scope: /app/. Only handles same-origin /app/ GETs; the IRC websocket and all
 // API calls (cloudflare, change_password, upload) pass straight through.
-const CACHE = 'tchatou-v50';
+const CACHE = 'tchatou-v51';
 const SHELL = ['/app/', '/app/index.html', '/app/favicon.svg', '/app/orbit-icon.svg', '/app/manifest.webmanifest'];
 
 self.addEventListener('install', (e) => {

src/config.ts

@@ -39,6 +39,7 @@ export interface AppConfig {
     sound: boolean;         // blip on mention / PM
     hideJoinQuit: boolean;  // hide join/part/quit noise
     clock24: boolean;       // 24h timestamps
+    lang?: string;          // force a default UI language (e.g. 'en'); omit to auto-detect from the browser
   };
   /** Feature switches — turn whole features off for a deployment. */
   features: {

src/i18n/index.ts

@@ -65,6 +65,18 @@ void i18n.use(initReactI18next).init({
 
 document.documentElement.lang = i18n.language;
 
+// Apply a deployment's default UI language (config.defaults.lang) — but only for
+// users who haven't explicitly picked one (setLang writes KEY). Called after the
+// runtime config loads, so it can override the browser-detected language.
+export function applyConfigDefaultLang(code?: string): void {
+  if (!code) return;
+  if (localStorage.getItem(KEY)) return;        // explicit user choice — honour it
+  if (!CODES.includes(code)) return;            // unknown language — ignore
+  if (i18n.language === code) return;
+  void i18n.changeLanguage(code);
+  document.documentElement.lang = code;
+}
+
 export function getLang(): string {
   return i18n.language;
 }

src/main.tsx

@@ -2,9 +2,10 @@ import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import { applyTheme, getTheme } from './ui/theme.ts'
 import './i18n'
+import { applyConfigDefaultLang } from './i18n'
 import './index.css'
 import { initViewport } from './ui/viewport.ts'
-import { loadConfig } from './config.ts'
+import { loadConfig, getConfig } from './config.ts'
 
 // Track the visual viewport so the layout shrinks above the on-screen keyboard.
 initViewport()
@@ -14,6 +15,7 @@ initViewport()
 // a rebuild.
 loadConfig().then(async () => {
   applyTheme(getTheme()) // re-apply now that config (default theme) is loaded
+  applyConfigDefaultLang(getConfig().defaults.lang) // honour a config-pinned default language
   const { default: App } = await import('./App.tsx') // also creates the store
   // Plugin subsystem: publish window.Orbit, bridge app/IRC events onto the bus,
   // then load operator-listed plugins from config. After the store exists,