Add a branded boot splash (Ladill Mail style) to app pages

InjectBootSplash middleware injects a full-screen loading splash — the app's
launcher icon (pulsing), an animated progress bar, and "Loading <app>…" — into
authenticated HTML pages, then fades it out on load. Shows once per browser
session (sessionStorage), so it masks the app's client-side boot without
flashing on every navigation. Self-branding from the app's own subdomain icon,
so the same partial/middleware drops into every Ladill app.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bootstrap/app.php

@@ -18,6 +18,7 @@ return Application::configure(basePath: dirname(__DIR__))
         ]));
         $middleware->web(append: [
             \App\Http\Middleware\SetActingAccount::class,
+            \App\Http\Middleware\InjectBootSplash::class,
         ]);
         $middleware->alias([
             'platform.session' => \App\Http\Middleware\EnsurePlatformSession::class,