feat: i18n system - French/English internationalization

Core: i18n.js module with t(), setLocale(), initI18n()
- Flat key→string JSON locale files (fr.json + en.json, 165 keys)
- data-i18n / data-i18n-placeholder / data-i18n-attr DOM attributes
- Live locale switch via setLocale() - no page reload
- Language detection: localStorage > navigator.language > 'fr'
- Fallback: missing key → French → raw key

Wired into:
- app.js: await initI18n() early in init()
- config.js: profile save → await setLocale(), toasts + confirm via t()
- auth.js: network error toast via t()
- index.html: data-i18n on login form, profile section, search placeholder

frontend/js/app.js

@@ -4,6 +4,7 @@
 
 import * as UI from './ui.js';
 import * as Utils from './utils.js';
+import { initI18n } from './i18n.js';
 
 // Wire up AI toolbar toast (avoids circular import in utils.js)
 window._obsigateShowToast = UI.showToast;
@@ -23,6 +24,7 @@ import { initMermaid, setupFocusMode } from './mermaid-viewer.js';
 // =========================================================================
 async function init() {
   UI.initTheme();
+  await initI18n();  // Must run before any rendering that uses t()
   UI.initHeaderMenu();
   UI.initCustomDropdowns();