feat: Implement LLM provider deletion and refactor ConfigManager to preserve unknown payload sections.

frontend/src/components/llm/ProviderConfig.svelte

@@ -31,6 +31,7 @@
   let testStatus = $state({ type: "", message: "" });
   let isTesting = $state(false);
   let togglingProviderIds = $state(new Set());
+  let deletingProviderIds = $state(new Set());
 
   function isMultimodalModel(modelName) {
     const token = (modelName || "").toLowerCase();
@@ -155,6 +156,38 @@
       onSave();
     }
   }
+
+  async function handleDelete(provider) {
+    if (deletingProviderIds.has(provider.id)) return;
+    if (
+      !confirm(
+        $t.llm.delete_confirm.replace("{name}", provider.name || provider.id),
+      )
+    ) {
+      return;
+    }
+
+    deletingProviderIds.add(provider.id);
+    deletingProviderIds = new Set(deletingProviderIds);
+
+    try {
+      await requestApi(`/llm/providers/${provider.id}`, "DELETE");
+      providers = providers.filter(({ id }) => id !== provider.id);
+      addToast($t.llm.delete_success.replace("{name}", provider.name), "success");
+      onSave();
+    } catch (err) {
+      addToast(
+        $t.llm.delete_failed.replace(
+          "{error}",
+          err?.message || $t.common.error,
+        ),
+        "error",
+      );
+    } finally {
+      deletingProviderIds.delete(provider.id);
+      deletingProviderIds = new Set(deletingProviderIds);
+    }
+  }
 </script>
 
 <div class="p-4">
@@ -345,11 +378,23 @@
           >
             {$t.common.edit}
           </button>
+          <button
+            type="button"
+            class="text-sm text-red-600 hover:underline"
+            onclick={() => handleDelete(provider)}
+            disabled={deletingProviderIds.has(provider.id)}
+          >
+            {#if deletingProviderIds.has(provider.id)}
+              ...
+            {:else}
+              {$t.common.delete}
+            {/if}
+          </button>
           <button
             type="button"
             class={`text-sm ${provider.is_active ? "text-orange-600" : "text-green-600"} hover:underline`}
             onclick={() => toggleActive(provider)}
-            disabled={togglingProviderIds.has(provider.id)}
+            disabled={togglingProviderIds.has(provider.id) || deletingProviderIds.has(provider.id)}
           >
             {#if togglingProviderIds.has(provider.id)}
               ...