feat(v5): add coold restart action

Add the V5 server endpoint and UI action to restart coold over SSH with
a fresh host token, plus Flux reconnect handling and coverage for the
new backend, browser, and canvas test suites.

tests/Pest.php

@@ -14,7 +14,7 @@ use Tests\TestCase;
 | need to change it using the "uses()" function to bind a different classes or traits.
 |
 */
-uses(TestCase::class)->in('Feature', 'v4/Feature', 'v4/Browser');
+uses(TestCase::class)->in('Feature', 'v4/Feature', 'v4/Browser', 'v5/Browser');
 
 /*
 |--------------------------------------------------------------------------