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>

assets/scss/partials/cookies.scss

@@ -0,0 +1,199 @@
+.cookie-banner {
+    position: fixed;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    z-index: 9999;
+    padding: var(--container-padding);
+
+    &[aria-hidden="true"] {
+        display: none;
+    }
+}
+
+.cookie-banner__content {
+    max-width: 900px;
+    margin: 0 auto;
+    background: var(--card-background);
+    border-radius: var(--card-border-radius);
+    box-shadow: var(--shadow-l3);
+    padding: var(--card-padding);
+    display: flex;
+    flex-direction: column;
+    gap: 15px;
+
+    @include respond(md) {
+        flex-direction: row;
+        flex-wrap: wrap;
+        align-items: center;
+        gap: 20px;
+    }
+}
+
+.cookie-banner__text {
+    flex: 1;
+    min-width: 200px;
+
+    strong {
+        display: block;
+        color: var(--card-text-color-main);
+        font-size: 1.6rem;
+        margin-bottom: 5px;
+    }
+
+    p {
+        color: var(--card-text-color-secondary);
+        font-size: 1.4rem;
+        line-height: 1.6;
+        margin: 0;
+    }
+}
+
+.cookie-banner__actions {
+    display: flex;
+    gap: 10px;
+    flex-shrink: 0;
+}
+
+.cookie-banner__settings-link {
+    width: auto;
+    align-self: flex-start;
+    background: none;
+    border: none;
+    color: var(--accent-color);
+    font-size: 1.3rem;
+    cursor: pointer;
+    padding: 5px;
+
+    &:hover {
+        text-decoration: underline;
+    }
+
+    @include respond(md) {
+        width: auto;
+        margin-left: auto;
+        align-self: center;
+    }
+}
+
+.cookie-btn {
+    padding: 10px 20px;
+    border-radius: var(--tag-border-radius);
+    font-size: 1.4rem;
+    font-weight: 500;
+    cursor: pointer;
+    transition: all 0.2s ease;
+    border: none;
+
+    &--primary {
+        background: var(--accent-color);
+        color: var(--accent-color-text);
+
+        &:hover {
+            background: var(--accent-color-darker);
+        }
+    }
+
+    &--secondary {
+        background: transparent;
+        color: var(--card-text-color-main);
+        border: 1.5px solid var(--card-separator-color);
+
+        &:hover {
+            background: var(--card-background-selected);
+        }
+    }
+}
+
+// Cookie settings panel
+.cookie-settings {
+    padding-top: 20px;
+    border-top: 1.5px solid var(--card-separator-color);
+
+    &[aria-hidden="true"] {
+        display: none;
+    }
+
+    h3 {
+        color: var(--card-text-color-main);
+        font-size: 1.6rem;
+        margin: 0 0 15px 0;
+    }
+
+    &__actions {
+        display: flex;
+        justify-content: flex-end;
+        gap: 10px;
+        margin-top: 20px;
+    }
+}
+
+.cookie-category {
+    padding: 15px 0;
+    border-bottom: 1px solid var(--card-separator-color);
+
+    &:last-of-type {
+        border-bottom: none;
+    }
+
+    label {
+        display: flex;
+        align-items: center;
+        gap: 10px;
+        cursor: pointer;
+
+        input[type="checkbox"] {
+            width: 18px;
+            height: 18px;
+            accent-color: var(--accent-color);
+            cursor: pointer;
+
+            &:disabled {
+                opacity: 0.6;
+                cursor: not-allowed;
+            }
+        }
+
+        strong {
+            color: var(--card-text-color-main);
+            font-size: 1.5rem;
+        }
+    }
+
+    p {
+        color: var(--card-text-color-secondary);
+        font-size: 1.3rem;
+        margin: 8px 0 0 28px;
+        line-height: 1.5;
+    }
+}
+
+// Footer link for reopening settings
+.cookie-settings-link {
+    display: inline-block;
+    color: var(--body-text-color);
+    font-size: 1.2rem;
+    cursor: pointer;
+    background: none;
+    border: none;
+    padding: 0;
+
+    &:hover {
+        color: var(--accent-color);
+        text-decoration: underline;
+    }
+}
+
+// Comments placeholder when consent not given
+.consent-placeholder {
+    background: var(--card-background);
+    border-radius: var(--card-border-radius);
+    padding: var(--card-padding);
+    text-align: center;
+    color: var(--card-text-color-secondary);
+    font-size: 1.4rem;
+
+    p {
+        margin: 0 0 15px 0;
+    }
+}

assets/scss/style.scss

@@ -23,6 +23,7 @@
 @import "partials/layout/list.scss";
 @import "partials/layout/404.scss";
 @import "partials/layout/search.scss";
+@import "partials/cookies.scss";
 
 @import "general.scss";
 @import "custom.scss";