Add Create Container UI and Templates (Unraid XML) feature

Lets users create containers from the WebUI (image/name/ports/volumes/
env/network mode/restart policy/privileged) instead of only managing
existing ones, and adds a Templates panel to save/reuse those configs
as Unraid-Docker-compatible template XML, including browsing and
importing the host's own existing Docker Manager templates directly.

Also fixes bugs found via live testing along the way: container names
with spaces/invalid characters now get a clear client- and server-side
error with a suggested fix instead of podman's raw API error, the New
Container modal's backdrop no longer renders transparent (was being
appended outside the .podman-plugin CSS scope), and modal buttons now
have real visual hierarchy (ghost/primary/danger) after Unraid's own
site-wide button theme was found to override plain single-class rules.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

webui/plugins/podman/ajax/containers.php

@@ -154,6 +154,19 @@ function build_container_spec(string $image, array $body): array
 
     $name = trim((string) ($body['name'] ?? ''));
     if ($name !== '') {
+        // Same character set podman itself enforces (define.NameRegex in
+        // libpod) — validated here too so a space/invalid character gets
+        // a clear message instead of podman's raw "running container
+        // create option: names must match ...: invalid argument" (found
+        // live: a template-derived container name with a space in it hit
+        // exactly this).
+        if (preg_match('/^[a-zA-Z0-9][a-zA-Z0-9_.-]*$/', $name) !== 1) {
+            podman_json_error(
+                "Container name (\"{$name}\") can only contain letters, digits, \".\", \"_\", \"-\" — no spaces. Try \"" .
+                preg_replace('/[^a-zA-Z0-9_.-]+/', '-', $name) . '" instead.',
+                400
+            );
+        }
         $spec['name'] = $name;
     }