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/auth.js

@@ -2,6 +2,7 @@
 import { state } from './state.js';
 import { safeCreateIcons } from './utils.js';
 import { showToast, closeHeaderMenu } from './ui.js';
+import { t } from './i18n.js';
 import { showWelcome } from './viewer.js';
 
 // ---------------------------------------------------------------------------
@@ -33,7 +34,7 @@ async function api(path, opts) {
     res = await fetch(path, mergedOpts);
   } catch (err) {
     if (err.name === "AbortError") throw err; // let callers handle abort
-    showToast("Erreur réseau — vérifiez votre connexion", "error");
+    showToast(t("auth.network_error"), "error");
     throw err;
   }
   if (res.status === 401 && AuthManager._authEnabled) {