feat: comprehensive i18n coverage — all static text + auto-translate toasts

Locale files expanded to 300+ keys covering all sections:
- Header menu, sidebar, search, viewer, editor, AI toolbar
- Config modal (all sections), help modal, dashboard, graph, mermaid
- Context menus, offline UI, palette, auth, shares, backups
- All toasts and confirmations

Automatic data-i18n tagging in index.html (177 attributes)
via Python script — handles all user-facing static text nodes.
Zero concatenation errors verified.

i18n.js: added tr() reverse-lookup function
(French text → key → current locale translation)

ui.js: showToast() now auto-translates via tr()
— all 24 existing toast call sites benefit automatically

frontend/js/ui.js

@@ -6,6 +6,7 @@ import { safeCreateIcons, getFileIcon, escapeHtml } from './utils.js';
 import { syncActiveFileTreeItem, refreshSidebarTreePreservingState } from './sidebar.js';
 import { GraphViewManager } from './graph.js';
 import { DashboardConflictsWidget } from './dashboard.js';
+import { t, tr } from './i18n.js';
 // ---------------------------------------------------------------------------
 // Right Sidebar Manager
 // ---------------------------------------------------------------------------
@@ -347,7 +348,8 @@ export function populateCustomDropdown(dropdownId, optionsList, defaultValue) {
 
 /** Display a brief toast message at the bottom of the viewport. */
 export function showToast(message, type) {
-  console.log("showToast called with:", message, type);
+  // Auto-translate French messages via reverse lookup
+  message = tr(message);
   type = type || "info";
   let container = document.getElementById("toast-container");
   if (!container) {