feat(i18n): add internationalization with DE, FR, ES, PT translations

Add a compile-time i18n system with 270 translation keys across 5 locales
(EN, DE, FR, ES, PT). Translations are embedded via include_str! and parsed
lazily into flat HashMaps with English fallback for missing keys.

- Add src/i18n module with Locale enum, t()/tw() lookup functions, and tests
- Add JSON translation files for all 5 locales under assets/i18n/
- Provide locale Signal via Dioxus context in App, persisted to localStorage
- Replace all hardcoded UI strings across 33 component/page files
- Add compact locale picker (globe icon + ISO alpha-2 code) in sidebar header
- Add click-outside backdrop dismissal for locale dropdown

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app.rs

@@ -1,3 +1,4 @@
+use crate::i18n::Locale;
 use crate::{components::*, pages::*};
 use dioxus::prelude::*;
 
@@ -61,8 +62,29 @@ const GOOGLE_FONTS: &str = "https://fonts.googleapis.com/css2?\
     display=swap";
 
 /// Root application component. Loads global assets and mounts the router.
+///
+/// Provides a `Signal<Locale>` context that all child components can read
+/// via `use_context::<Signal<Locale>>()` to access the current locale.
+/// The locale is persisted in `localStorage` under `"certifai_locale"`.
 #[component]
 pub fn App() -> Element {
+    // Read persisted locale from localStorage on first render.
+    let initial_locale = {
+        #[cfg(feature = "web")]
+        {
+            web_sys::window()
+                .and_then(|w| w.local_storage().ok().flatten())
+                .and_then(|s| s.get_item("certifai_locale").ok().flatten())
+                .map(|code| Locale::from_code(&code))
+                .unwrap_or_default()
+        }
+        #[cfg(not(feature = "web"))]
+        {
+            Locale::default()
+        }
+    };
+    use_context_provider(|| Signal::new(initial_locale));
+
     rsx! {
         // Seggwat feedback widget
         document::Script {