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/skills.ts

@@ -0,0 +1,55 @@
+import { request } from './client'
+
+export interface SkillInfo {
+  name: string
+  description: string
+}
+
+export interface SkillCategory {
+  name: string
+  description: string
+  skills: SkillInfo[]
+}
+
+export interface SkillListResponse {
+  categories: SkillCategory[]
+}
+
+export interface SkillFileEntry {
+  path: string
+  name: string
+  isDir: boolean
+}
+
+export interface MemoryData {
+  memory: string
+  user: string
+  memory_mtime: number | null
+  user_mtime: number | null
+}
+
+export async function fetchSkills(): Promise<SkillCategory[]> {
+  const res = await request<SkillListResponse>('/api/skills')
+  return res.categories
+}
+
+export async function fetchSkillContent(skillPath: string): Promise<string> {
+  const res = await request<{ content: string }>(`/api/skills/${skillPath}`)
+  return res.content
+}
+
+export async function fetchSkillFiles(category: string, skill: string): Promise<SkillFileEntry[]> {
+  const res = await request<{ files: SkillFileEntry[] }>(`/api/skills/${category}/${skill}/files`)
+  return res.files
+}
+
+export async function fetchMemory(): Promise<MemoryData> {
+  return request<MemoryData>('/api/memory')
+}
+
+export async function saveMemory(section: 'memory' | 'user', content: string): Promise<void> {
+  await request('/api/memory', {
+    method: 'POST',
+    body: JSON.stringify({ section, content }),
+  })
+}