feat(splashscreen): implement a splash screen for application startup

This commit introduces a splash screen that enhances the user experience during application startup. It includes a new HTML file for the splash screen layout and a corresponding icon. The main application logic has been updated to close the splash screen once the main application is ready.

- Added a splash screen HTML and icon for improved visual feedback
- Updated main application logic to manage splash screen visibility
- Enhanced title bar branding with an image instead of SVG

src/app.css

@@ -477,8 +477,8 @@
   .titlebar-brand {
     display: flex;
     align-items: center;
-    gap: 7px;
-    padding: 0 14px;
+    gap: 8px;
+    padding: 0 13px 0 11px;
     height: 100%;
     font-size: 13px;
     font-weight: 700;
@@ -486,7 +486,19 @@
     color: var(--color-bar-text);
     border-right: 1px solid rgba(255,255,255,0.06);
   }
-  .titlebar-brand svg { color: #ffd343; filter: drop-shadow(0 0 10px rgba(255,211,67,0.3)); flex-shrink: 0; }
+  .titlebar-brand-icon {
+    display: grid;
+    place-items: center;
+    width: 24px;
+    height: 24px;
+    flex: 0 0 24px;
+  }
+  .titlebar-brand-icon img {
+    width: 24px;
+    height: 24px;
+    object-fit: contain;
+    filter: drop-shadow(0 1px 3px rgba(0,0,0,0.5));
+  }
 
   .tb-version {
     margin-left: 1px;