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/components/app_shell.rs

@@ -1,6 +1,7 @@
 use dioxus::prelude::*;
 
 use crate::components::sidebar::Sidebar;
+use crate::i18n::{t, tw, Locale};
 use crate::infrastructure::auth_check::check_auth;
 use crate::models::AuthInfo;
 use crate::Route;
@@ -12,6 +13,8 @@ use crate::Route;
 /// sidebar with real user data and the active child route.
 #[component]
 pub fn AppShell() -> Element {
+    let locale = use_context::<Signal<Locale>>();
+
     // use_resource memoises the async call and avoids infinite re-render
     // loops that use_effect + spawn + signal writes can cause.
     #[allow(clippy::redundant_closure)]
@@ -40,16 +43,17 @@ pub fn AppShell() -> Element {
             nav.push(NavigationTarget::<Route>::External("/auth".into()));
             rsx! {
                 div { class: "app-shell loading",
-                    p { "Redirecting to login..." }
+                    p { {t(*locale.read(), "auth.redirecting_login")} }
                 }
             }
         }
         Some(Err(e)) => {
             let msg = e.to_string();
+            let error_text = tw(*locale.read(), "auth.auth_error", &[("msg", &msg)]);
             rsx! {
                 div { class: "auth-error",
-                    p { "Authentication error: {msg}" }
-                    a { href: "/auth", "Login" }
+                    p { {error_text} }
+                    a { href: "/auth", {t(*locale.read(), "common.login")} }
                 }
             }
         }
@@ -57,7 +61,7 @@ pub fn AppShell() -> Element {
             // Still loading.
             rsx! {
                 div { class: "app-shell loading",
-                    p { "Loading..." }
+                    p { {t(*locale.read(), "common.loading")} }
                 }
             }
         }