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/javascript/app.js

@@ -162,7 +162,7 @@ window.Podman = (function () {
       '<div class="podman-modal-head"><h3>' + escapeHtml(opts.title) + '</h3></div>' +
       '<form class="podman-modal-body">' + fieldsHtml + '</form>' +
       '<div class="podman-modal-actions">' +
-      '<button type="button" class="podman-btn" data-role="cancel">Cancel</button>' +
+      '<button type="button" class="podman-btn podman-btn-ghost" data-role="cancel">Cancel</button>' +
       '<button type="button" class="podman-btn podman-btn-primary" data-role="submit">' +
       escapeHtml(opts.submitLabel || 'Create') + '</button>' +
       '</div></div>';