Add customer logo area to action bar

Introduced a customer logo section in `EnvelopeSenderPage.razor` that dynamically displays a customer logo or a placeholder when no logo is configured. The logo is fetched from the `CustomImagesService` and styled using new CSS classes in `sender-page.css`.

- Added `_companyLogo` property to store customer logo configuration.
- Updated `CustomImagesService` to fetch both app and customer logos.
- Modified `appsettings.json` to include logo paths and styles.
- Enhanced CSS for a three-section action bar layout.
- Added placeholder styling for missing customer logos.
- Updated initialization logic to load logos dynamically.
- Documented implementation details in `SF-74-PLAN.md`.

The implementation is modular and deployment-specific, allowing logo paths and styles to be updated via configuration without code changes.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/wwwroot/css/sender-page.css

@@ -28,6 +28,34 @@
     display: flex;
     align-items: center;
     gap: 1rem;
+    flex-shrink: 0;     /* don't shrink — logo area in the middle gets the space */
+}
+
+/* ── Customer logo area (middle of action bar) ──────────────────────────── */
+.sender-customer-logo-area {
+    flex: 1;                            /* takes up remaining space between left and right */
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    min-width: 0;                       /* allow flex child to shrink below content size */
+}
+
+/* Placeholder shown when no customer logo is configured */
+.sf-customer-logo-placeholder {
+    display: flex;
+    align-items: center;
+    height: 28px;
+    min-width: 90px;
+    padding: 0 12px;
+    border: 1px dashed rgba(126, 34, 206, 0.28);
+    border-radius: 6px;
+    font-size: 0.65rem;
+    font-weight: 500;
+    color: rgba(126, 34, 206, 0.38);
+    white-space: nowrap;
+    letter-spacing: 0.03em;
+    text-transform: uppercase;
+    user-select: none;
 }
 
 .sender-logo svg {