feat: add model selector, skills/memory pages, and config management

- Add model selector in sidebar that discovers models from auth.json credential pool
- Add per-session model display (badge in chat header and session list)
- Add skills browser page and memory editor page
- Add BFF routes for skills, memory, and config model management
- Model switching updates config.yaml provider field to bypass env auto-detection
- Refactor Settings page, simplify ChatInput with file upload
- Add attachment upload support via BFF /upload endpoint

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

src/api/system.ts

@@ -16,6 +16,41 @@ export interface ModelsResponse {
   data: Model[]
 }
 
+// Config-based model types
+export interface ModelInfo {
+  id: string
+  label: string
+}
+
+export interface ModelGroup {
+  provider: string
+  models: ModelInfo[]
+}
+
+export interface ConfigModelsResponse {
+  default: string
+  groups: ModelGroup[]
+}
+
+export interface AvailableModelGroup {
+  provider: string   // credential pool key (e.g. "zai", "custom:subrouter.ai")
+  label: string      // display name (e.g. "zai", "subrouter.ai")
+  base_url: string
+  models: string[]
+}
+
+export interface AvailableModelsResponse {
+  default: string
+  groups: AvailableModelGroup[]
+}
+
+export interface CustomProvider {
+  name: string
+  base_url: string
+  api_key: string
+  model: string
+}
+
 export async function checkHealth(): Promise<HealthResponse> {
   return request<HealthResponse>('/health')
 }
@@ -23,3 +58,36 @@ export async function checkHealth(): Promise<HealthResponse> {
 export async function fetchModels(): Promise<ModelsResponse> {
   return request<ModelsResponse>('/v1/models')
 }
+
+export async function fetchConfigModels(): Promise<ConfigModelsResponse> {
+  return request<ConfigModelsResponse>('/api/config/models')
+}
+
+export async function fetchAvailableModels(): Promise<AvailableModelsResponse> {
+  return request<AvailableModelsResponse>('/api/available-models')
+}
+
+export async function updateDefaultModel(data: {
+  default: string
+  provider?: string
+  base_url?: string
+  api_key?: string
+}): Promise<void> {
+  await request('/api/config/model', {
+    method: 'PUT',
+    body: JSON.stringify(data),
+  })
+}
+
+export async function addCustomProvider(data: CustomProvider): Promise<void> {
+  await request('/api/config/providers', {
+    method: 'POST',
+    body: JSON.stringify(data),
+  })
+}
+
+export async function removeCustomProvider(name: string): Promise<void> {
+  await request(`/api/config/providers/${encodeURIComponent(name)}`, {
+    method: 'DELETE',
+  })
+}