feat: add GDPR cookie consent banner (#1216)

* feat: add GDPR cookie consent banner

Add a GDPR-compliant cookie consent banner that gates analytics (Google
Analytics) and functional cookies (comments) until user consent is given.

Features:
- Theme-consistent styling using CSS variables (light/dark mode)
- Settings panel with category toggles (necessary, analytics, functional)
- Consent stored in cookie for 365 days
- Google Analytics only loads after analytics consent
- Comments only load after functional consent
- Translations for all 32 supported languages
- Accessible with proper ARIA attributes and focus management

Configuration in hugo.yaml:
  params:
    cookies:
      enabled: true
      categories:
        analytics: true
        functional: true

When cookies.enabled is false, scripts load normally without consent gating.

Closes #412

* refactor: move consent-gated GA to cookies/analytics.html

Move Google Analytics consent logic to avoid shadowing Hugo's internal
`_internal/google_analytics.html` template.

Changes:
- Rename google_analytics.html → cookies/analytics.html
- Update head.html to conditionally load:
  - cookies.enabled=true: cookies/analytics.html (consent-gated)
  - cookies.enabled=false: Hugo's _internal/google_analytics.html
- Align cookies/analytics.html with Hugo's internal template:
  - Support Privacy.GoogleAnalytics.Disable setting
  - Support Privacy.GoogleAnalytics.RespectDoNotTrack setting
  - Add UA- prefix deprecation warning

Note: Custom JS is required for consent-gated GA because Hugo templates
render at build time, but consent happens at runtime. We cannot call
Hugo's internal template after the user grants consent - we must
dynamically inject the GA script via JavaScript.

* style: indent cookies/analytics.html

* refactor: don't use default, put default configurations in params.toml

* fix: don't use `description` as translation key because it's reserved and will throw error

* fix: remove footer/components/custom-font.html call

* fix: banner showSettings key

* style: indent comments/include

* Add missing article.alert translation back

* style: format i18n toml files

* style: indent head/head.html

* style adjustment

* style: add missing -

* style: format demo/config/params.toml

* style: remove redundant comments from cookies.scss

* style: update box-shadow for cookie banner content

---------

Co-authored-by: delize <4028612+delize@users.noreply.github.com>
Co-authored-by: Jimmy Cai <jimmy@cai.im>

layouts/_partials/comments/include.html

@@ -1,3 +1,47 @@
 {{ if .Site.Params.comments.enabled }}
-    {{ partial (printf "comments/provider/%s" .Site.Params.comments.provider) . }}
+    {{- $needsConsent := and .Site.Params.cookies.enabled .Site.Params.cookies.categories.functional -}}
+    {{- if $needsConsent -}}
+        {{/* Consent-gated comments - show placeholder until functional consent */}}
+        <div id="comments-consent-placeholder" class="consent-placeholder">
+            <p>{{ T "cookies.commentsDisabled" }}</p>
+            <button class="cookie-btn cookie-btn--primary" data-cookie-action="reopen">
+                {{ T "cookies.managePreferences" }}
+            </button>
+        </div>
+        <div id="comments-container" style="display: none;">
+            {{ partial (printf "comments/provider/%s" .Site.Params.comments.provider) . }}
+        </div>
+        <script>
+        (function() {
+            var placeholder = document.getElementById('comments-consent-placeholder');
+            var container = document.getElementById('comments-container');
+
+            function showComments() {
+                if (placeholder) placeholder.style.display = 'none';
+                if (container) container.style.display = 'block';
+            }
+
+            function hideComments() {
+                if (placeholder) placeholder.style.display = 'block';
+                if (container) container.style.display = 'none';
+            }
+
+            window.addEventListener('onCookieConsentChange', function(e) {
+                if (e.detail && e.detail.functional) {
+                    showComments();
+                } else {
+                    hideComments();
+                }
+            });
+
+            // Check if already consented
+            if (window.cookieConsent && window.cookieConsent.hasConsent('functional')) {
+                showComments();
+            }
+        })();
+        </script>
+    {{- else -}}
+        {{/* No consent required - load comments normally */}}
+        {{ partial (printf "comments/provider/%s" .Site.Params.comments.provider) . }}
+    {{- end -}}
 {{ end }}