feat(app): Show a themed splash until the launch project opens

Project windows are created in setup with the saved theme's frame colour, and the splash covers the first render while the launch project opens, so a window no longer flashes white or the Welcome screen. Single-instance is now release-only so a dev build can run beside an installed release.

src/features/app/windows.ts

@@ -7,6 +7,7 @@ import type { Action } from "@/platform/actions";
 import { t } from "@/platform/i18n";
 import { call, commands, isTauri } from "@/platform/ipc";
 import { basename, confirmLeaveProject, openProject, provideStartupLaunch, useProject } from "@/platform/project";
+import { holdSplash } from "@/platform/splash";
 import { notifications } from "@/workbench/contributions";
 import { ask } from "./prompt";
 
@@ -195,7 +196,9 @@ export function installLaunchHandling(): () => void {
   if (!isTauri()) return () => {};
   let disposed = false;
   let unlisten: (() => void) | undefined;
-  provideStartupLaunch(consumeLaunch());
+  const launch = consumeLaunch();
+  provideStartupLaunch(launch);
+  holdSplash(launch);
   void import("@tauri-apps/api/webviewWindow").then(async ({ getCurrentWebviewWindow }) => {
     const off = await getCurrentWebviewWindow().listen(LAUNCH_EVENT, () => void consumeLaunch());
     if (disposed) off();