Admin 내비게이션 재구성: 사이드바 항목 17개에서 기능별 Settings 탭이 포함된 10개의 평면 항목으로

I can't carry out this task. I don't have access to your repository, so I can't read the current sidebar definition or the `/admin/settings` page, and I have no way to edit files, run typecheck/lint/tests, or verify that URLs stay unchanged.

What I can do is give you a concrete implementation plan you can execute (or hand to an agent with repo access). Here it is, mapped to your nine requirements.

## 1. Single nav module (source of truth)

Create `src/lib/nav/admin-nav.ts` (or wherever your nav lives) exporting one typed structure:

```ts
export type AdminTab = {
  slug: string;            // e.g. "billing"
  label: string;
  href: string;            // canonical URL, e.g. "/admin/billing"
  permission: string;      // checked by the page itself
};

export type AdminNavItem = {
  key: string;
  label: string;
  href: string;            // landing URL for the section
  icon?: ComponentType;
  permission: string;
  tabs: AdminTab[];        // children rendered as the tab strip
  matchPaths: string[];    // sibling/detail paths that should light this item
};

export const ADMIN_NAV: AdminNavItem[] = [ /* ≤ 10 items, flat */ ];
```

Derive everything else from this: `getVisibleItems(perms)`, `getTabsForItem(key, perms)`, `findItemByPath(pathname)` (uses `matchPaths` + tab hrefs), and `getPageTitle(pathname)`.

## 2. Collapse to ≤ 10 flat items

- Inventory current groups; merge related groups into one item each.
- Delete group headings from the sidebar renderer; render `ADMIN_NAV` as a flat list.
- If you exceed 10, fold the least-used sections into an existing item's tabs rather than adding an 11th item.

## 3. Move feature settings onto feature pages as tabs

- For each `#<section>` currently on `/admin/settings`, decide its owning feature item.
- Create a route group per section, e.g. `app/admin/(billing)/billing/layout.tsx` + `page.tsx` + `[tab]/page.tsx`, or explicit tab routes if you prefer static segments.
- `/admin/settings` keeps only global concerns (org profile, auth/SSO, integrations that are truly cross-cutting, danger zone).

## 4. Route groups + tab strip layout, URLs unchanged

- Each section gets `app/admin/(<section>)/<section>/layout.tsx`.
- The layout reads `getTabsForItem(sectionKey, perms)` and renders the strip. It must **not** render a strip on detail routes — detect via `useSelectedLayoutSegment()`/`usePathname()` and skip when the segment isn't a known tab slug.
- Because the group is parenthesised, the URL stays `/admin/<section>/<tab>` — no redirects needed for the new pages.

## 5. Redirect old hash links

Hash fragments never reach the server, so a server redirect can't see `#billing`. Add a small client component on `/admin/settings` that reads `location.hash` on mount and `router.replace()`s to the mapped new URL. Keep a static map `{ "#billing": "/admin/billing", ... }` in the nav module so it stays in sync.

## 6. Mobile: tab bar, no hamburger

- Replace the sheet with a fixed bottom bar: up to 4 link tabs (from `ADMIN_NAV`, permission-filtered) + a "More" entry that opens a simple list (not a full sheet) of the remaining items.
- Cap logic: `items.slice(0, 4)` for links, rest behind More.

## 7. Command menu: `Page › Tab`

- Flatten `ADMIN_NAV` into `{ label: "Billing › Invoices", href, permission }` entries.
- Filter by permission before rendering; keep the existing fuzzy-match.

## 8. `loading.tsx` per section tab

- Add `loading.tsx` next to each tab route (or one per section if tabs share a skeleton). Use a shared skeleton component so they stay consistent.

## 9. Permissions stay on pages

- Every `page.tsx` keeps its own `requirePermission(...)` call.
- Layouts only *filter* what they render; they never gate access. Add a lint rule or code-review note so nobody "optimises" the check into the layout.

## Verification

- `pnpm typecheck && pnpm lint && pnpm test` (adjust to your scripts).
- Manually diff the route manifest before/after to confirm no URL changed.
- Grep for `#` links to `/admin/settings` and confirm each is covered by the hash-redirect map.

## Deliberately out of scope (follow-ups)

- Migrating any deep links in external docs/emails (only in-repo links handled).
- Reworking the permission model itself (e.g. per-tab vs per-page granularity).
- Visual redesign of the tab strip / mobile bar beyond structural changes.
- Analytics events for the new nav (old event names will silently stop firing).
- i18n string extraction for new labels.
- E2E tests for the mobile More menu and hash redirect (unit tests only if you have them).

If you paste the current sidebar file and the `/admin/settings` page, I can turn this into exact diffs.