refactor: saasMode 분기 제거 — 항상 SaaS(OAuth only)로 동작

Mac/Windows 환경 불일치 해소. BUILD_TIME 상수, CONFIGURE 채널,
개발자 모드 UI 전부 제거. Supabase 설정은 supabase-config.ts SSOT.

apps/desktop/src/preload/index.ts

@@ -715,8 +715,6 @@ const electronAPI = {
         lastSyncAt: number | null
         syncing: boolean
       }>(IPC_CHANNELS.CLOUD_SYNC.GET_STATE),
-    configure: (params: { url: string; anonKey: string }) =>
-      invoke<unknown>(IPC_CHANNELS.CLOUD_SYNC.CONFIGURE, params),
     signIn: (params: { provider: 'google' | 'github' }) =>
       invoke<{ started: boolean }>(IPC_CHANNELS.CLOUD_SYNC.SIGN_IN, params),
     signOut: () => invoke<unknown>(IPC_CHANNELS.CLOUD_SYNC.SIGN_OUT),