Improve logo rendering and page responsiveness

Updated `EnvelopeSenderPage.razor` to enhance customer logo rendering with dynamic height and class styling. Refined the "Nur ausgewählte" checkbox label for better styling.

Modified `sender-page.css` to improve layout and responsiveness:
- Added styles for `.sender-customer-logo-area` and `.signflow-company-logo` for proper scaling and alignment.
- Removed `.sender-export-label` styles and introduced `.sender-export-selected-text` for improved checkbox label design.
- Adjusted media queries for better layout on screens up to 1320px and 768px.

Enhanced overall responsiveness and accessibility of the sender page.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderPage.razor

@@ -51,7 +51,7 @@
                 @if (_companyLogo is not null && !string.IsNullOrEmpty(_companyLogo.Src))
                 {
                     <img src="@_companyLogo.Src"
-                         style="height: @_companyLogo.Height"
+                         style="--sf-company-logo-height: @_companyLogo.Height"
                          class="@_companyLogo.GetClass("Show")"
                          alt="Kunden-Logo" />
                 }
@@ -105,7 +105,6 @@
                 </button>
 
                 <div class="sender-export-group" title="Export der aktuellen Tabellenansicht">
-                    <label class="sender-export-label" for="sender-export-format">Format</label>
                     <select id="sender-export-format" class="sender-export-format" @bind="_selectedExportFormat" aria-label="Exportformat auswählen">
                         <option value="xlsx">XLSX</option>
                         <option value="csv">CSV</option>
@@ -114,7 +113,10 @@
 
                     <label class="sender-export-selected">
                         <input type="checkbox" @bind="_exportSelectedRowsOnly" />
-                        <span>Nur ausgewählte</span>
+                        <span class="sender-export-selected-text">
+                            <span>Nur</span>
+                            <span>ausgewählte</span>
+                        </span>
                     </label>
 
                     <button class="sender-btn sender-btn--export"