i18n: fix hardcoded French in Statistics and Shortcuts panel

- dashboard.js: Fichiers/Tags uniques/Taille totale/Vaults -> t() calls
- config.js: Tags uniques/Vaults in About -> t() calls
- index.html: Raccourcis & Astuces -> data-i18n
- Added 5 new keys: dashboard.stats_*, help.shortcuts_title

frontend/index.html

@@ -999,7 +999,7 @@
                                             data-lucide="keyboard"
                                             style="width: 16px; height: 16px"
                                         ></i>
-                                        <span>Raccourcis &amp; Astuces</span>
+                                        <span data-i18n="help.shortcuts_title">Raccourcis &amp; Astuces</span>
                                         <button
                                             class="qh-toggle"
                                             id="qh-toggle"