Refactor web build process and update documentation

- Removed Node.js build artifacts from .gitignore and adjusted Makefile to reflect changes in web UI build process, now using server-rendered Go templates instead of React.
- Updated README to clarify the new web UI architecture and output formats, emphasizing the removal of the Node.js build step.
- Added a command to set the number of frames per render task in manager configuration, enhancing user control over rendering settings.
- Improved Gitea workflow by removing unnecessary npm install step, streamlining the CI process.

web/templates/admin.html

@@ -0,0 +1,49 @@
+{{ define "page_admin" }}
+{{ $view := .Data }}
+<section class="card">
+  <h1>Admin Panel</h1>
+  <div class="check-row">
+    <label>
+      <input id="registration-enabled" type="checkbox" {{ if index $view "registration_enabled" }}checked{{ end }}>
+      Allow new registrations
+    </label>
+    <button id="save-registration" class="btn">Save</button>
+  </div>
+</section>
+
+<section class="card">
+  <h2>Runners</h2>
+  <div id="admin-runners"
+       hx-get="/ui/fragments/admin/runners"
+       hx-trigger="load, every 6s"
+       hx-swap="innerHTML">
+    <p>Loading runners...</p>
+  </div>
+</section>
+
+<section class="card">
+  <h2>Users</h2>
+  <div id="admin-users"
+       hx-get="/ui/fragments/admin/users"
+       hx-trigger="load, every 10s"
+       hx-swap="innerHTML">
+    <p>Loading users...</p>
+  </div>
+</section>
+
+<section class="card">
+  <div class="section-head">
+    <h2>Runner API Keys</h2>
+    <button id="create-api-key" class="btn">Create API Key</button>
+  </div>
+  <div id="admin-apikeys"
+       hx-get="/ui/fragments/admin/apikeys"
+       hx-trigger="load, every 10s"
+       hx-swap="innerHTML">
+    <p>Loading API keys...</p>
+  </div>
+</section>
+
+<p id="admin-message" class="alert notice hidden"></p>
+<p id="admin-error" class="alert error hidden"></p>
+{{ end }}