Add DE/EN language toggle with bilingual descriptions

- Language switcher in sidebar (DE/EN buttons, persists to localStorage)
- i18n module with pick_desc/pick_name helpers for language-aware fallback
- All detail/list pages use language context for names and descriptions
- Species/Cultivar types updated with description_de/description_en
- Common Name column added to species/families lists

herbapi-ui/src/app.rs

@@ -1,8 +1,13 @@
 use dioxus::prelude::*;
+use gloo_storage::{LocalStorage, Storage};
 
 use crate::api;
 use crate::types::MeResponse;
 
+/// Global language signal shared via context. Values: "de" or "en".
+#[derive(Clone, Copy)]
+pub struct Lang(pub Signal<String>);
+
 #[derive(Routable, Clone, Debug, PartialEq)]
 #[rustfmt::skip]
 pub enum Route {
@@ -43,6 +48,15 @@ pub fn App() -> Element {
 
 #[component]
 fn Layout() -> Element {
+    // Language state: load from localStorage, default "de"
+    let lang_signal = use_signal(|| {
+        LocalStorage::get::<String>("herbapi_lang").unwrap_or_else(|_| "de".to_string())
+    });
+    use_context_provider(|| Lang(lang_signal));
+
+    let mut lang = use_context::<Lang>().0;
+    let current_lang = lang.read().clone();
+
     // Try to get current user (may be None for public access)
     let auth = use_resource(|| async { api::get_current_user().await.ok() });
     let user: Option<MeResponse> = auth.read().as_ref().and_then(|r| r.clone());
@@ -66,6 +80,26 @@ fn Layout() -> Element {
                     NavLink { to: Route::SearchPage {}, label: "Search" }
                     NavLink { to: Route::Sources {}, label: "Sources" }
                 }
+                div { class: "sidebar-lang",
+                    div { class: "lang-toggle",
+                        button {
+                            class: if current_lang == "de" { "lang-btn lang-btn-active" } else { "lang-btn" },
+                            onclick: move |_| {
+                                lang.set("de".to_string());
+                                let _ = LocalStorage::set("herbapi_lang", "de".to_string());
+                            },
+                            "DE"
+                        }
+                        button {
+                            class: if current_lang == "en" { "lang-btn lang-btn-active" } else { "lang-btn" },
+                            onclick: move |_| {
+                                lang.set("en".to_string());
+                                let _ = LocalStorage::set("herbapi_lang", "en".to_string());
+                            },
+                            "EN"
+                        }
+                    }
+                }
                 div { class: "sidebar-user",
                     if let Some(ref u) = user {
                         span { class: "user-name", "{u.nickname.as_deref().or(u.name.as_deref()).unwrap_or(&u.email)}" }