feat(model-server): wizard V2 config modèles locaux — source, options structurées & preview

Refonte UX du wizard de configuration des modèles locaux du serveur
llama.cpp intégré, alignée sur la source Hugging Face et les options backend.

- choix de source sans jargon (chemin local .gguf vs dépôt Hugging Face) ;
- champs structurés -ngl (gpu_layers), -c (context_size), --jinja, --host ;
- zone d'arguments libres avec alerte sur les flags réservés ;
- preview de la commande via le backend (previewModelServerCommand, debounced) ;
- gateway/ports et mock adaptés au contrat V2.

QA : Vitest 22 verts, tsc/build OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/model-servers/useModelServers.ts

@@ -11,7 +11,11 @@
 
 import { useCallback, useEffect, useState } from "react";
 
-import type { GatewayError, LocalModelServerConfig } from "@/domain";
+import type {
+  GatewayError,
+  LocalModelServerConfig,
+  ModelServerCommandPreview,
+} from "@/domain";
 import { useGateways } from "@/app/di";
 
 /** What the model-server UI needs from this hook. */
@@ -28,6 +32,14 @@ export interface ModelServersViewModel {
   save: (config: LocalModelServerConfig) => Promise<LocalModelServerConfig | null>;
   /** Deletes a server by id; returns `true` on success. */
   remove: (serverId: string) => Promise<boolean>;
+  /**
+   * Asks the backend to build the `llama-server` command line for a draft
+   * (never reconstructed client-side). Returns `null` when the draft is
+   * invalid — the editor shows a placeholder instead of a stale command.
+   */
+  preview: (
+    config: LocalModelServerConfig,
+  ) => Promise<ModelServerCommandPreview | null>;
   /** Clears the current error banner. */
   clearError: () => void;
 }
@@ -120,7 +132,20 @@ export function useModelServers(): ModelServersViewModel {
     [modelServer],
   );
 
+  const preview = useCallback(
+    async (config: LocalModelServerConfig) => {
+      try {
+        return await modelServer.previewModelServerCommand(config);
+      } catch {
+        // A preview is best-effort UI sugar: an invalid draft (e.g. no source)
+        // simply yields no command, without touching the CRUD error banner.
+        return null;
+      }
+    },
+    [modelServer],
+  );
+
   const clearError = useCallback(() => setError(null), []);
 
-  return { servers, error, busy, reload, save, remove, clearError };
+  return { servers, error, busy, reload, save, remove, preview, clearError };
 }