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.

phpunit.xml

@@ -10,6 +10,9 @@
     <testsuite name="v4">
       <directory suffix="Test.php">./tests/v4</directory>
     </testsuite>
+    <testsuite name="v5">
+      <directory suffix="Test.php">./tests/v5</directory>
+    </testsuite>
   </testsuites>
   <coverage/>
   <php>