terms: per-term installer visibility toggles + universal delete (v0.3.4)

- _meta.json: customLabels -> terms.{label,showInInstaller,showInInstallerRp}
- Drop builtin protection; any term kind can be deleted/added/toggled
- New public route /manifest/terms/<pack>/index.json for installer term lists
- Installers fetch terms:list dynamically; skip agreement step if list empty
- Term editor: 2 visibility checkboxes (설치기 / 리소스팩 설치기), multi-select
- Migration from old schema preserves custom labels (default: visible in both)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/installer/preload.ts

@@ -14,6 +14,9 @@ const api = {
   // 약관(Markdown) 다운로드
   getTerm: (kind: string): Promise<{ ok: boolean; content?: string; message?: string }> =>
     ipcRenderer.invoke('terms:get', kind),
+  // 메인 인스톨러용 약관 목록 (사이트의 visibility 토글에 따라 필터링됨)
+  getTermsList: (): Promise<{ ok: boolean; terms?: Array<{ kind: string; label: string }>; message?: string }> =>
+    ipcRenderer.invoke('terms:list'),
 
   // 3-1
   pickFolder: (): Promise<string | null> => ipcRenderer.invoke('dialog:pickFolder'),