webapp: add /onboard wizard — web equivalent of php artisan rig:loading

Browser-based Project Loading manifest builder at http://127.0.0.1:1118/onboard.
Reuses the existing terminal/CRT styling (hud.css palette + scanlines +
phosphor glow) so it doesn't feel like a separate UI:

 - GET  /onboard           — the wizard, seeded from current loading.json
 - GET  /onboard/search?q= — type-ahead Twitch search/categories proxy
 - POST /onboard           — writes storage/data/loading.json, optionally
                              pushes the title + category via TwitchHelix

UX: prompt-line inputs, ↑/↓/Enter on the search dropdown, stepper for the
countdown, ENABLED/DISABLED toggles for camera + microphone, a live
right-pane preview that renders the same lines the loading overlay's
terminal will type. CSRF-exempt POST so the page can submit JSON without
a token round-trip.

Falls back gracefully when TWITCH_CLIENT_ID / TWITCH_TOKEN are absent —
the search is disabled and the manifest still saves locally.

webapp/bootstrap/app.php

@@ -12,6 +12,7 @@ return Application::configure(basePath: dirname(__DIR__))
     ->withMiddleware(function (Middleware $middleware): void {
         $middleware->validateCsrfTokens(except: [
             'cmd/*',
+            'onboard',
         ]);
     })
     ->withExceptions(function (Exceptions $exceptions): void {