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/assets/herbapi.css

@@ -120,8 +120,51 @@ em {
     text-decoration: none;
 }
 
-.sidebar-user {
+/* Language toggle */
+
+.sidebar-lang {
     margin-top: auto;
+    padding: 0.75rem 1.25rem;
+    border-top: 1px solid rgba(255,255,255,0.1);
+    display: flex;
+    justify-content: center;
+}
+
+.lang-toggle {
+    display: flex;
+    border-radius: 4px;
+    overflow: hidden;
+    border: 1px solid rgba(255,255,255,0.2);
+}
+
+.lang-btn {
+    padding: 0.3rem 0.75rem;
+    background: transparent;
+    color: rgba(255,255,255,0.5);
+    border: none;
+    font-size: 0.75rem;
+    font-weight: 600;
+    letter-spacing: 0.5px;
+    cursor: pointer;
+    transition: all 0.15s;
+}
+
+.lang-btn:hover {
+    color: rgba(255,255,255,0.8);
+    background: rgba(255,255,255,0.05);
+}
+
+.lang-btn-active {
+    background: var(--accent);
+    color: #fff;
+}
+
+.lang-btn-active:hover {
+    background: var(--accent-hover);
+    color: #fff;
+}
+
+.sidebar-user {
     padding: 1rem 1.25rem;
     border-top: 1px solid rgba(255,255,255,0.1);
     display: flex;