feat: add model management module with provider CRUD

- New /models page with provider list (built-in + custom)
- Add provider via preset selection or custom URL with auto-fetch models
- Delete provider removes from auth.json credential_pool + config.yaml custom_providers
- Auto-switch model on add, fallback switch on delete
- Sync sidebar ModelSelector on all provider changes
- Unified provider presets in shared/providers.ts (frontend + backend)
- Backend uses hardcoded catalog first, live probe as fallback

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/views/ModelsView.vue

@@ -0,0 +1,84 @@
+<script setup lang="ts">
+import { ref, onMounted } from 'vue'
+import { NButton, NSpin } from 'naive-ui'
+import ProvidersPanel from '@/components/models/ProvidersPanel.vue'
+import ProviderFormModal from '@/components/models/ProviderFormModal.vue'
+import { useModelsStore } from '@/stores/models'
+
+const modelsStore = useModelsStore()
+const showModal = ref(false)
+
+onMounted(() => {
+  modelsStore.fetchProviders()
+})
+
+function openCreateModal() {
+  showModal.value = true
+}
+
+function handleModalClose() {
+  showModal.value = false
+}
+
+async function handleSaved() {
+  await modelsStore.fetchProviders()
+  handleModalClose()
+}
+</script>
+
+<template>
+  <div class="models-view">
+    <header class="models-header">
+      <h2 class="header-title">Models</h2>
+      <NButton type="primary" @click="openCreateModal">
+        <template #icon>
+          <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><line x1="12" y1="5" x2="12" y2="19"/><line x1="5" y1="12" x2="19" y2="12"/></svg>
+        </template>
+        Add Provider
+      </NButton>
+    </header>
+
+    <div class="models-content">
+      <NSpin :show="modelsStore.loading && modelsStore.providers.length === 0">
+        <ProvidersPanel />
+      </NSpin>
+    </div>
+
+    <ProviderFormModal
+      v-if="showModal"
+      @close="handleModalClose"
+      @saved="handleSaved"
+    />
+  </div>
+</template>
+
+<style scoped lang="scss">
+@use '@/styles/variables' as *;
+
+.models-view {
+  height: 100vh;
+  display: flex;
+  flex-direction: column;
+}
+
+.models-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 12px 20px;
+  border-bottom: 1px solid $border-color;
+  flex-shrink: 0;
+}
+
+.header-title {
+  font-size: 16px;
+  font-weight: 600;
+  color: $text-primary;
+}
+
+.models-content {
+  flex: 1;
+  overflow-y: auto;
+  padding: 20px;
+}
+</style>