fix: i18n live-reload for dashboard — tabs now respect language setting

Root cause: dashboard tabs rendered via t() template literals had no
data-i18n attributes, so setLocale() _applyDOM() could not re-translate
them live. User switching to English in profile saw no effect without
page reload.

Changes:
- i18n.js: expose t/initI18n/setLocale/getLocale/tr globally (window.*)
  for non-module script compatibility
- viewer.js: add data-i18n attributes to all 4 dashboard tabs + loading
  text + empty state messages (12 elements total)

Verified: tabs show 'Statistics/Bookmarks/Recent/Shared' in EN mode

frontend/js/i18n.js

@@ -138,6 +138,13 @@ async function _loadLocaleStrings(locale) {
   }
 }
 
+// ── Global exports for non-module scripts (legacy app.js) ──────────────────
+window.t = t;
+window.tr = tr;
+window.getLocale = getLocale;
+window.setLocale = setLocale;
+window.initI18n = initI18n;
+
 /** Walk the DOM and update every element with a data-i18n* attribute. */
 function _applyDOM() {
   // textContent