Add Title and Message fields to Envelope editor

Introduced input fields for "Title" and "Message" in the
EnvelopeSenderEditorPage, allowing users to specify metadata
for envelopes. The "Title" field is required and validated,
while the "Message" field is optional with a default value.

Updated the save logic to validate the title and display
appropriate error messages. Persisted the "Title" and
"Message" fields in the session cache via the
`EditorSessionData` record.

Modified the `CreateEnvelopeReceiverCommand` to use the
user-provided "Title" and "Message" values. Adjusted the UI
layout and styling to accommodate the new fields, ensuring
a seamless user experience.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderEditorPage.razor

@@ -28,14 +28,55 @@
         <div class="envelope-action-bar__inner"
              style="flex-direction: row; align-items: center; padding: 0.35rem 1.5rem; gap: 0.75rem;">
 
-            @* Left: Title *@
+            @* Left: Title + meta inputs + receivers *@
-            <div style="flex: 1; min-width: 0; display: flex; flex-direction: column; align-items: stretch; gap: 0.75rem;">
+            <div style="flex: 1; min-width: 0; display: flex; flex-direction: column; align-items: stretch; gap: 0.5rem;">
-                <div style="display: flex; align-items: center; gap: 0.75rem; min-width: 0; flex-wrap: wrap;">
+
-                    <div style="font-size: 0.9rem; font-weight: 600; color: #1f2937; white-space: nowrap;">
+                @* ── Title & Message inputs ── *@
-                        Neues Dokument
+                <div style="display: flex; align-items: flex-start; gap: 0.5rem; flex-wrap: wrap;">
+
+                    @* Titel *@
+                    <div style="display: flex; flex-direction: column; gap: 0.2rem; flex: 1 1 180px; min-width: 140px;">
+                        <label style="font-size: 0.65rem; font-weight: 600; color: #6b7280;
+                                      text-transform: uppercase; letter-spacing: 0.05em;">
+                            Titel <span style="color:#dc2626;">*</span>
+                        </label>
+                        <input type="text"
+                               value="@_envelopeTitle"
+                               @oninput="e => _envelopeTitle = e.Value?.ToString() ?? string.Empty"
+                               placeholder="z.B. Arbeitsvertrag 2025"
+                               maxlength="120"
+                               style="font-size: 0.78rem; font-weight: 500; color: #1f2937;
+                                      border: 1px solid #d1d5db; border-radius: 5px;
+                                      padding: 0.3rem 0.55rem; outline: none; width: 100%;
+                                      background: #fff;
+                                      @(_titleTouched && string.IsNullOrWhiteSpace(_envelopeTitle) ? "border-color:#dc2626;" : "")" />
+                        @if (_titleTouched && string.IsNullOrWhiteSpace(_envelopeTitle))
+                        {
+                            <span style="font-size:0.6rem;color:#dc2626;">Pflichtfeld</span>
+                        }
                     </div>
+
+                    @* Nachricht *@
+                    <div style="display: flex; flex-direction: column; gap: 0.2rem; flex: 2 1 260px; min-width: 180px;">
+                        <label style="font-size: 0.65rem; font-weight: 600; color: #6b7280;
+                                      text-transform: uppercase; letter-spacing: 0.05em;">
+                            Nachricht <span style="color:#6b7280;font-weight:400;">(optional)</span>
+                        </label>
+                        <input type="text"
+                               value="@_envelopeMessage"
+                               @oninput="e => _envelopeMessage = e.Value?.ToString() ?? string.Empty"
+                               placeholder="Bitte unterzeichnen Sie das beigefügte Dokument."
+                               maxlength="500"
+                               style="font-size: 0.78rem; color: #1f2937;
+                                      border: 1px solid #d1d5db; border-radius: 5px;
+                                      padding: 0.3rem 0.55rem; outline: none; width: 100%;
+                                      background: #fff;" />
+                    </div>
+
+                    @* PDF filename badge *@
                     @if (_pdfLoaded)
                     {
+                        <div style="display:flex;align-items:flex-end;gap:0.4rem;padding-bottom:0.05rem;flex-shrink:0;">
                             <span style="font-size: 0.7rem; color: #6b7280;">@_fileName</span>
                             @if (_signatureFields.Count > 0)
                             {
@@ -45,6 +86,7 @@
                                     @_signatureFields.Count Signaturfeld@(_signatureFields.Count != 1 ? "er" : "")
                                 </span>
                             }
+                        </div>
                     }
                 </div>
 
@@ -419,6 +461,11 @@
     bool    _savePopupVisible  = false;
     string? _saveErrorMessage  = null;
 
+    // ── Envelope metadata ──
+    string _envelopeTitle   = string.Empty;
+    string _envelopeMessage = string.Empty;
+    bool   _titleTouched    = false;
+
     List<ReceiverDraft> _receivers = [];
     bool   _receiverPopupVisible;
     string _receiverDraftName        = string.Empty;
@@ -469,6 +516,8 @@
             _fileName         = cached.FileName;
             _pdfLoaded        = _originalPdfBytes is { Length: > 0 };
             _receivers        = cached.Receivers;
+            _envelopeTitle    = cached.Title;
+            _envelopeMessage  = cached.Message;
 
             // Redraw placeholders onto the original PDF
             if (_pdfLoaded)
@@ -625,12 +674,19 @@
     async Task SaveAsync()
     {
         // ── Validation ──
+        _titleTouched = true;
         if (!_pdfLoaded || _originalPdfBytes is null)
         {
             _saveErrorMessage = "Bitte laden Sie zuerst ein PDF-Dokument hoch.";
             _savePopupVisible = true;
             return;
         }
+        if (string.IsNullOrWhiteSpace(_envelopeTitle))
+        {
+            _saveErrorMessage = "Bitte geben Sie einen Titel ein.";
+            _savePopupVisible = true;
+            return;
+        }
         if (_receivers.Count == 0)
         {
             _saveErrorMessage = "Bitte fügen Sie mindestens einen Empfänger hinzu.";
@@ -690,8 +746,10 @@
 
             var command = new CreateEnvelopeReceiverCommand
             {
-                Title      = "Neuer Umschlag",       // placeholder — dedicated field will be added later
+                Title      = _envelopeTitle.Trim(),
-                Message    = "Bitte unterzeichnen Sie das beigefügte Dokument.",
+                Message    = string.IsNullOrWhiteSpace(_envelopeMessage)
+                                 ? "Bitte unterzeichnen Sie das beigefügte Dokument."
+                                 : _envelopeMessage.Trim(),
                 TFAEnabled = false,
                 Document   = new DocumentCreateCommand { DataAsBase64 = docBase64 },
                 Receivers  = receiversCmd,
@@ -728,7 +786,9 @@
             OriginalPdfBytes: _originalPdfBytes ?? [],
             Fields:           [.. _signatureFields],
             FileName:         _fileName,
-            Receivers:        [.. _receivers]);
+            Receivers:        [.. _receivers],
+            Title:            _envelopeTitle,
+            Message:          _envelopeMessage);
 
         MemoryCache.Set(SessionKey, data, SessionTtl);
     }
@@ -977,7 +1037,9 @@
         byte[]                   OriginalPdfBytes,
         List<SignatureFieldDraft> Fields,
         string                   FileName,
-        List<ReceiverDraft>      Receivers);
+        List<ReceiverDraft>      Receivers,
+        string                   Title,
+        string                   Message);
 
     // ── Receiver colour palette (cycles when > 8 receivers) ──
     static readonly string[] ReceiverPalette =