feat: add SPA Blade shell and catch-all route

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/Feature/SpaRouteTest.php

@@ -0,0 +1,23 @@
+<?php
+
+it('serves the spa shell for the root path', function (): void {
+    $response = $this->get('/');
+
+    $response->assertStatus(200);
+    $response->assertSee('<div id="app">', false);
+});
+
+it('serves the spa shell for unknown frontend paths', function (): void {
+    $response = $this->get('/some/frontend/route');
+
+    $response->assertStatus(200);
+    $response->assertSee('<div id="app">', false);
+});
+
+it('does not intercept api routes', function (): void {
+    $response = $this->get('/api/stations');
+
+    // API route handles it (403 from missing key, not SPA HTML)
+    $response->assertStatus(403);
+    $response->assertJson(['message' => '']);
+});