feat: i18n string externalization with en + vi bundles

- Move every theme-rendered string (nav aria-label, theme-toggle labels, default footer template) to i18n/{lang}.toml.
- Ship en (default) + vi bundles. Adding a language is a single-file change.
- Hugo selects bundle via defaultContentLanguage (not languageCode); documented in README.
- All i18n calls have | default fallbacks so missing keys never break rendering.

themes/bonsai/i18n/en.toml

@@ -0,0 +1,14 @@
+# Bonsai theme — English strings.
+# Override per language by adding e.g. i18n/vi.toml; missing keys fall back to en.
+
+[nav_links_label]
+other = "Links"
+
+[theme_toggle_label]
+other = "Toggle light and dark theme"
+
+[theme_toggle_title]
+other = "Toggle theme"
+
+[footer_default]
+other = "© {{ .year }} {{ .name }}"

themes/bonsai/i18n/vi.toml

@@ -0,0 +1,13 @@
+# Bonsai theme — Vietnamese strings.
+
+[nav_links_label]
+other = "Liên kết"
+
+[theme_toggle_label]
+other = "Chuyển giao diện sáng / tối"
+
+[theme_toggle_title]
+other = "Đổi giao diện"
+
+[footer_default]
+other = "© {{ .year }} {{ .name }}"

themes/bonsai/layouts/partials/footer.html

@@ -3,7 +3,8 @@
   {{- with site.Params.footerText -}}
     <p>{{ . | safeHTML }}</p>
   {{- else -}}
-    <p>© {{ now.Year }} {{ site.Params.name | default site.Title }}</p>
+    {{- $ctx := dict "year" now.Year "name" (site.Params.name | default site.Title) -}}
+    <p>{{ i18n "footer_default" $ctx | default (printf "© %d %s" now.Year (site.Params.name | default site.Title)) | safeHTML }}</p>
   {{- end -}}
   {{- if site.Params.themeToggle -}}
     {{ partial "theme-toggle-button.html" . }}

themes/bonsai/layouts/partials/theme-toggle-button.html

@@ -2,9 +2,9 @@
        Sun shown in dark mode (target = light). Moon shown in light mode (target = dark). */ -}}
 <button type="button" class="theme-toggle"
         data-bonsai-theme-toggle
-        aria-label="Toggle light and dark theme"
+        aria-label="{{ i18n "theme_toggle_label" | default "Toggle light and dark theme" }}"
         aria-pressed="false"
-        title="Toggle theme">
+        title="{{ i18n "theme_toggle_title" | default "Toggle theme" }}">
   <svg class="theme-toggle__sun" viewBox="0 0 24 24" width="18" height="18" aria-hidden="true">
     <circle cx="12" cy="12" r="4" fill="currentColor"/>
     <g stroke="currentColor" stroke-width="2" stroke-linecap="round">