Add dedicated Companion Planting page

Backend: new GET /api/v1/companions endpoint returning all companion
relationships with joined species names, slugs and images. Adds
CompanionWithNames model and list_all DB query.

Frontend: new /companions route with search bar, beneficial (green)
and antagonistic (red) sections, species thumbnails, mechanism text,
source links, and species detail links. Full DE/EN i18n support.

herbapi-ui/src/app.rs

@@ -31,6 +31,8 @@ pub enum Route {
         SupplierList {},
         #[route("/suppliers/:slug")]
         SupplierDetail { slug: String },
+        #[route("/companions")]
+        CompanionList {},
         #[route("/search")]
         SearchPage {},
         #[route("/sources")]
@@ -80,6 +82,7 @@ fn Layout() -> Element {
                     NavLink { to: Route::SpeciesList {}, label: t(l, "nav.species") }
                     NavLink { to: Route::CultivarList {}, label: t(l, "nav.cultivars") }
                     NavLink { to: Route::SupplierList {}, label: t(l, "nav.suppliers") }
+                    NavLink { to: Route::CompanionList {}, label: t(l, "nav.companions") }
                     NavLink { to: Route::SearchPage {}, label: t(l, "nav.search") }
                     NavLink { to: Route::Sources {}, label: t(l, "nav.sources") }
                 }
@@ -140,6 +143,7 @@ fn NotFound(segments: Vec<String>) -> Element {
 }
 
 // Re-export page components for the router
+pub use crate::pages::companions::CompanionList;
 pub use crate::pages::cultivars::{CultivarDetail, CultivarList};
 pub use crate::pages::families::{FamilyDetail, FamilyList};
 pub use crate::pages::home::Home;