feat(web): dark Gitea UI, systemd installer, WEB-UI docs (W7)

Restyles every template with a hand-written dark Gitea-style theme
(web/static/app.css, ~270 lines: badges, buttons, cards, focus rings,
prefers-reduced-motion) and completes the htmx polling wiring for all
four dashboard cards. Adds the repeatable setup-web-linux.sh installer
(idempotent: pip install '[web]' non-editable, render ci-web.service,
template [web]/[web.oidc] into config.toml, prompt for keyring secrets,
print Gitea OIDC registration steps, enable/restart the unit) plus
deploy/systemd/ci-web.service and docs/WEB-UI.md.

src/ci_orchestrator/web/templates/dashboard.html

@@ -3,13 +3,9 @@
 {% block title %}Dashboard - ci-orchestrator{% endblock %}
 
 {% block content %}
-<section class="card" id="vms-card">
+<div class="dashboard-grid">
   {% include "partials/vms.html" %}
-</section>
-
-<section class="card" id="cache-card">
   {% include "partials/cache.html" %}
-</section>
-
-{% include "partials/host.html" %}
+  {% include "partials/host.html" %}
+</div>
 {% endblock %}