feat(ui): themed startup splash with deferred window show (#1030)

* feat(ui): add themed startup splash with deferred window show

Show a theme-aware loading splash before the Vite bundle mounts, hide the
native window until it paints, and use per-theme logo gradient colors.

* docs: note startup splash in CHANGELOG and credits for PR #1030

* docs: attribute PR #1030 startup splash to cucadmuh

src/app/bootstrap.ts

@@ -1,4 +1,5 @@
 import { installQueueUndoHotkey } from '../store/queueUndoHotkey';
+import { configureStartupSplash } from './startupSplash';
 import { invoke } from '@tauri-apps/api/core';
 import { getWindowKind } from './windowKind';
 import { migrateThemeSelection } from '../utils/themes/themeMigration';
@@ -112,6 +113,7 @@ export function runPreReactBootstrap(): void {
   migrateThemeSelection();
   // Paint the correct theme on the very first frame (no Mocha flash).
   applyThemeAtStartup();
+  configureStartupSplash();
   installCrossWindowThemeSync();
   markDevBuildDocument();
   pushUserAgentToBackend();