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>

tests/Feature/PosRegisterTest.php

@@ -50,6 +50,12 @@ class PosRegisterTest extends TestCase
             ->assertSee('favicon.ico', false);
     }
 
+    public function test_boot_splash_injected_on_authenticated_pages(): void
+    {
+        $this->actingAs($this->user())->get(route('pos.dashboard'))
+            ->assertOk()->assertSee('id="ladill-boot"', false);
+    }
+
     public function test_register_renders_local_products_in_restaurant_mode(): void
     {
         $user = $this->user();