feat: enhance Mini App design guidelines and refactor layout components

- Updated Mini App design guidelines to include detailed instructions on UI changes, accessibility rules, and verification processes.
- Refactored multiple components to utilize `MiniAppScreen` and `MiniAppScreenContent` for consistent layout structure across the application.
- Improved error handling in `GlobalError` and `NotFound` components by integrating new layout components for better user experience.
- Introduced new hooks for admin functionality, streamlining access checks and data loading processes.
- Enhanced documentation to reflect changes in design policies and component usage, ensuring clarity for future development.

webapp-next/src/store/slices/session-slice.ts

@@ -0,0 +1,21 @@
+export interface SessionSlice {
+  lang: "ru" | "en";
+  /** True when the first visible screen has finished loading; used to hide content until ready(). */
+  appContentReady: boolean;
+  /** True when GET /api/admin/me returned is_admin: true; used to show Admin link. */
+  isAdmin: boolean;
+  setLang: (v: "ru" | "en") => void;
+  setAppContentReady: (v: boolean) => void;
+  setIsAdmin: (v: boolean) => void;
+}
+
+type SessionSet = (updater: Partial<SessionSlice> | ((state: SessionSlice) => Partial<SessionSlice>)) => void;
+
+export const createSessionSlice = (set: SessionSet): SessionSlice => ({
+  lang: "en",
+  appContentReady: false,
+  isAdmin: false,
+  setLang: (v) => set({ lang: v }),
+  setAppContentReady: (v) => set({ appContentReady: v }),
+  setIsAdmin: (v) => set({ isAdmin: v }),
+});