feat(i18n): add internationalization with DE, FR, ES, PT translations (#12)

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>

Co-authored-by: Sharang Parnerkar <parnerkarsharang@gmail.com>
Reviewed-on: #12

src/components/sub_nav.rs

@@ -9,7 +9,7 @@ use dioxus::prelude::*;
 /// * `route` - Route to navigate to when clicked
 #[derive(Clone, PartialEq)]
 pub struct SubNavItem {
-    pub label: &'static str,
+    pub label: String,
     pub route: Route,
 }