feat: add full-guide language toggle

Document the client-side, dual-rendered locale contract and dispatch a narrow language-change event for guide selector panels.\n\nDo not assemble the full-guide page or change its content collections; task 15d owns that integration.
2026-09-05 17:44:20 +00:00
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+# Context: full-guide language switching
+
+## Decision
+
+The Astro full guide keeps the current client-side language switch on its
+existing `/full-guide/` URL. It server-renders both locale variants and the
+language-toggle island shows the selected variant after `client:idle` hydration.
+
+This deliberately preserves the current no-URL-change contract, including links
+shared without a locale segment, and avoids a route/redirect and publishing
+change. The cost is duplicated localized HTML and both locales in the response.
+That is acceptable for this small guide and avoids sending duplicated string
+data through every interactive island.
+
+## Markup and island contract for task 15d
+
+- Render each static localized fragment twice. Put `data-language-content="en"`
+  or `data-language-content="pt"` on its outer element. English is visible in
+  server HTML; the toggle uses the native `hidden` attribute for the inactive
+  locale.
+- Add `<LanguageToggle />` to the guide top bar. Astro's `client:idle` directive
+  is only valid for framework components; this `.astro` island defers its
+  browser setup with `requestIdleCallback` (and a timeout fallback) instead. Do
+  not hydrate the page or use `client:load`; the control is deliberately
+  idle-priority.
+- The island owns the `ai-for-dummies-language` localStorage key. Every read and
+  write remains inside `try`/`catch`, because previews may disable storage.
+- On each selection the island sets `<html lang>` to `en` or `pt-BR`, updates
+  its `[data-lang]` buttons' `.active` class and `aria-pressed` state, updates
+  `[data-language-content]`, then dispatches `ai-for-dummies:languagechange` on
+  `window`. The event detail is `{ language: 'en' | 'pt' }`.
+- The guide selector island (15a) must read `document.documentElement.lang` when
+  it hydrates and listen for that event. On receipt it must re-render the
+  currently active phase and all active selector panels from their collection
+  data. This preserves today’s `applyLanguage` behaviour without coupling the
+  toggle to page selectors.
+
+This is a page-local contract: the existing `/rules/` toggle continues using its
+own `rules-language` key and must not be changed as part of full-guide
+migration.