Implement en/fr localization across admin and user UIs

Introduce client-side localization helper and en/fr locale bundles.

Route admin control, picks, movies, stats, winners and settings UIs through i18n keys instead of hardcoded text.

Remove French text from the English UI and standardize shared messages (saving, network errors, confirmations).

Update .gitignore to ignore editor metadata and localization helper artifacts.

public/js/app_theme.js

@@ -7,6 +7,21 @@
   const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
   const API_ENDPOINT = '/api/users/theme';
 
+  function t(key, fallback, params) {
+    try {
+      if (window.i18n && typeof window.i18n.t === 'function') {
+        return window.i18n.t(key, params || {});
+      }
+    } catch (_) {}
+    let out = (typeof fallback === 'string') ? fallback : String(key);
+    if (params && typeof params === 'object') {
+      Object.keys(params).forEach((k) => {
+        out = out.replace(new RegExp(`\\{${k}\\}`, 'g'), String(params[k]));
+      });
+    }
+    return out;
+  }
+
   function normalizeTheme(value) {
     return value === 'light' || value === 'dark' ? value : null;
   }
@@ -111,9 +126,14 @@
     buttons.forEach((button) => {
       button.setAttribute('aria-checked', String(isDark));
       button.dataset.theme = theme;
-      const label = isDark ? 'Mode sombre' : 'Mode clair';
-      const sourceLabel = source === 'system' ? ' (système)' : '';
-      button.setAttribute('title', `${label}${sourceLabel} — Shift+clic pour système`);
+      const label = isDark
+        ? t('theme.darkMode', 'Dark mode')
+        : t('theme.lightMode', 'Light mode');
+      const sourceLabel = source === 'system'
+        ? t('theme.systemSuffix', ' (system)')
+        : '';
+      const hint = t('theme.systemHint', '— Shift+click for system');
+      button.setAttribute('title', `${label}${sourceLabel} ${hint}`.trim());
     });
   }