chore: release v2.4

- Add theme system (Dark/Light palettes, ModernTheme, ThemeManager)
- Add InputDialog, Spinner common view
- Add DuplicatesCsvExportService
- Refresh views, dialogs, and view models across tabs
- Update localization strings (en/fr)
- Tweak services (transfer, permissions, search, user access, ownership elevation, bulk operations)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SharepointToolbox/Services/Export/ExportFileWriter.cs

@@ -0,0 +1,27 @@
+using System.IO;
+using System.Text;
+
+namespace SharepointToolbox.Services.Export;
+
+/// <summary>
+/// Central file-write plumbing for export services so every CSV and HTML
+/// artefact gets a consistent encoding: CSV files are written with a UTF-8
+/// BOM (required for Excel to detect the encoding when opening a
+/// double-clicked .csv), HTML files are written without a BOM (some browsers
+/// and iframe <c>srcdoc</c> paths render the BOM as a visible character).
+/// Export services should call these helpers rather than constructing
+/// <see cref="UTF8Encoding"/> inline.
+/// </summary>
+internal static class ExportFileWriter
+{
+    private static readonly UTF8Encoding Utf8WithBom = new(encoderShouldEmitUTF8Identifier: true);
+    private static readonly UTF8Encoding Utf8NoBom   = new(encoderShouldEmitUTF8Identifier: false);
+
+    /// <summary>Writes <paramref name="csv"/> to <paramref name="filePath"/> as UTF-8 with BOM.</summary>
+    public static Task WriteCsvAsync(string filePath, string csv, CancellationToken ct)
+        => File.WriteAllTextAsync(filePath, csv, Utf8WithBom, ct);
+
+    /// <summary>Writes <paramref name="html"/> to <paramref name="filePath"/> as UTF-8 without BOM.</summary>
+    public static Task WriteHtmlAsync(string filePath, string html, CancellationToken ct)
+        => File.WriteAllTextAsync(filePath, html, Utf8NoBom, ct);
+}