feat: implement per-member reading progress tracking for shared books (#30)

Backend (apps/groups):
- ReadingGroup model: named groups linked to EBooks with creator tracking
- GroupMembership model: user-group association with member/admin roles
- MemberProgress model: per-member progress with section tracking,
  percentage, time_spent, privacy toggles, and device-position preservation
- ReadingGroupViewSet: CRUD, join/leave, members list, progress endpoints
- GET /api/groups/{id}/members/progress/ — all members' progress
  (public: section label only; own: full detail; private: stubbed)
- PATCH /api/groups/{id}/progress/ — update own progress with auto-
  percentage calculation from section boundaries
- GET /api/groups/{id}/progress/summary/ — admin dashboard with
  averages, started/finished counts, and per-member details
- Permissions: IsGroupMember, IsGroupAdmin

Frontend:
- Groups list page (/groups) with create modal and book selector
- Group detail page (/groups/:id) with progress and admin summary tabs
- Progress bar visualization per member with privacy-aware display
- Admin stat cards (total members, started, finished, avg progress, avg time)
- Navigation link from Library header

Shared:
- ReadingGroupSummary and MemberProgressPublic types
- API endpoint constants for groups routes

Closes #30

packages/shared/src/types.ts

@@ -215,4 +215,36 @@ export interface PaginatedResponse<T> {
 export interface ApiError {
   detail?: string;
   [key: string]: unknown;
+}
+
+// ---- Reading Groups ----
+
+export interface ReadingGroupSummary {
+  id: number;
+  name: string;
+  ebook: number;
+  ebook_title: string;
+  ebook_author: string;
+  created_by_email: string;
+  description: string;
+  member_count: number;
+  my_progress: {
+    current_section: number;
+    percentage: number;
+    time_spent_seconds: number;
+    section_label: string;
+  } | null;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface MemberProgressPublic {
+  user_id: number;
+  user_email: string;
+  current_section: number;
+  total_sections: number;
+  section_label: string;
+  percentage: number;
+  time_spent_seconds: number;
+  updated_at: string;
 }

packages/shared/src/utils.ts

@@ -114,4 +114,13 @@ export const API_ENDPOINTS = {
     bookmarkDetail: (id: string) => `/api/annotations/bookmarks/${id}/`,
     noteDetail: (id: string) => `/api/annotations/notes/${id}/`,
   },
+  groups: {
+    list: "/api/groups/",
+    detail: (id: number) => `/api/groups/${id}/`,
+    membersProgress: (id: number) => `/api/groups/${id}/members/progress/`,
+    myProgress: (id: number) => `/api/groups/${id}/progress/`,
+    progressSummary: (id: number) => `/api/groups/${id}/progress/summary/`,
+    join: (id: number) => `/api/groups/${id}/join/`,
+    leave: (id: number) => `/api/groups/${id}/leave/`,
+  },
 } as const;