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/stores/models.ts

@@ -0,0 +1,78 @@
+import { defineStore } from 'pinia'
+import { ref, computed } from 'vue'
+import * as systemApi from '@/api/system'
+import type { AvailableModelGroup, CustomProvider } from '@/api/system'
+import { useAppStore } from './app'
+
+export const useModelsStore = defineStore('models', () => {
+  const providers = ref<AvailableModelGroup[]>([])
+  const defaultModel = ref('')
+  const loading = ref(false)
+
+  const customProviders = computed(() =>
+    providers.value.filter(g => g.provider.startsWith('custom:')),
+  )
+
+  const builtinProviders = computed(() =>
+    providers.value.filter(g => !g.provider.startsWith('custom:')),
+  )
+
+  const allModels = computed(() =>
+    providers.value.flatMap(g =>
+      g.models.map(m => ({
+        id: m,
+        provider: g.provider,
+        label: g.label,
+        base_url: g.base_url,
+        isDefault: m === defaultModel.value,
+      })),
+    ),
+  )
+
+  async function fetchProviders() {
+    loading.value = true
+    try {
+      const res = await systemApi.fetchAvailableModels()
+      providers.value = res.groups
+      defaultModel.value = res.default
+    } catch (err) {
+      console.error('Failed to fetch providers:', err)
+    } finally {
+      loading.value = false
+    }
+  }
+
+  async function setDefaultModel(modelId: string, provider: string) {
+    await systemApi.updateDefaultModel({ default: modelId, provider })
+    defaultModel.value = modelId
+    const appStore = useAppStore()
+    appStore.loadModels()
+  }
+
+  async function addProvider(data: CustomProvider) {
+    await systemApi.addCustomProvider(data)
+    await fetchProviders()
+    const appStore = useAppStore()
+    appStore.loadModels()
+  }
+
+  async function removeProvider(name: string) {
+    await systemApi.removeCustomProvider(name)
+    await fetchProviders()
+    const appStore = useAppStore()
+    appStore.loadModels()
+  }
+
+  return {
+    providers,
+    defaultModel,
+    loading,
+    customProviders,
+    builtinProviders,
+    allModels,
+    fetchProviders,
+    setDefaultModel,
+    addProvider,
+    removeProvider,
+  }
+})