Refactor API client and remove unused code

- Removed the ApiError class and related functions from the api-client module.
- Updated vendor detail page to handle errors without the ApiError class.
- Refactored various API modules to use in-memory mock data instead of commented-out fetch implementations.
- Removed the auth-token module as it was no longer needed.
- Updated error handling to use a more generic error structure.
- Adjusted common types to include ApiResult interface for better type management.

Frontend/PROGRESS.md

@@ -5,14 +5,18 @@ Spec: `docs/20-FRONTEND.md` (flows + rules) · `docs/11-BACKEND-PHASE1.md` (API
 Convention: `docs/01-DOC-GUIDE.md §6`. Update this file in the **same commit** as the code. When ticking `[x]`, append a short note + any deviation.
 
 ## 0. Foundation
-- [x] `NEXT_PUBLIC_API_BASE_URL` wired (`.env.local` / `.env.local.example`, alongside the existing `NEXT_PUBLIC_AUTH_API_BASE_URL`) — points at `Backend/ERPCore`'s https profile (`7112`)
-- [x] Typed API client / fetch wrapper (`lib/api-client.ts`: `apiRequest`/`apiRequestWithETag`/`buildQuery`) + bearer token handling (`lib/auth-token.ts`, degrades gracefully — no login endpoint wired yet, see §1)
-- [x] Shared TS types mirroring API DTOs (`types/common.ts`, `types/master-data.ts`, `types/procurement.ts`, `types/grn.ts`) — built only for the subset GRN needs (Item/Warehouse/Bin/Vendor/Uom/PurchaseOrder/Grn)
+- [x] `NEXT_PUBLIC_API_BASE_URL` wired (`.env.local` / `.env.local.example`, alongside the existing `NEXT_PUBLIC_AUTH_API_BASE_URL`) — currently unused now that the fetch client is gone (see 2026-07-15 note below)
+- [ ] Typed API client / fetch wrapper — **removed 2026-07-15** (`lib/api-client.ts` + `lib/auth-token.ts` deleted per explicit user request: "delete all api connections"). No live code path in the frontend makes an HTTP call to any backend anymore — every screen is 100% mock data (`lib/api/mock-data.ts`).
+- [x] Shared TS types mirroring API DTOs (`types/common.ts`, `types/master-data.ts`, `types/procurement.ts`, `types/grn.ts`) — built only for the subset GRN needs (Item/Warehouse/Bin/Vendor/Uom/PurchaseOrder/Grn); `types/common.ts` now also carries `ApiResult<T>` (moved here 2026-07-15 when `lib/api-client.ts` was deleted, since it's a plain data envelope, not fetch-specific)
 - [~] Client validation helpers (`lib/validations/grn.ts`) — **deviation**: uses `zod` (already a project dependency, used by `lib/validations.ts`/login), not hand-rolled, to stay consistent with the codebase's existing pattern rather than introduce a second validation approach. UX-only; server remains authoritative (docs/20-FRONTEND.md §3)
-- [x] `ProblemDetails` normalizer (`ApiError` in `lib/api-client.ts`) + `code → message` map (`lib/error-map.ts`)
+- [x] `code → message` map (`lib/error-map.ts`) — `errorMessage`/`fieldErrors` duck-type any `{ code, detail, errors }`-shaped rejection (the `ApiError` class they used to check via `instanceof` no longer exists); this also fixed a latent bug where the mock layer's plain `Error`-plus-`.code` rejects never matched the old `instanceof ApiError` check, so `CODE_MESSAGES` silently never applied to any mock error
 
 > **Scope note:** this foundation was built alongside the GRN feature and only covers the endpoints GRN consumes (items, warehouses/bins, vendors, uoms, purchase-orders, grns). Other master-data/procurement API methods still need their own `lib/api/*.ts` files when those screens are built.
 
+> **2026-07-15 — fetch infrastructure deleted outright (not just reverted to mock).** Following an earlier same-week pass that wired every `lib/api/*.ts` module to real `fetch` calls (then reverted via `git revert --no-commit` at the user's request — see `Backend`-adjacent history if relevant later), the user asked to go further and delete the underlying connection mechanism entirely, not just leave it unused. Deleted `lib/api-client.ts` (`apiRequest`/`apiRequestWithETag`/`buildQuery`/`ApiError`) and `lib/auth-token.ts` (bearer-token storage) as files. Follow-on fixes this required: (1) `ApiResult<T>` — used by `items.ts`/`purchase-orders.ts`/`vendors.ts` for their mock ETag pattern — moved into `types/common.ts`; (2) `lib/error-map.ts` rewritten to duck-type instead of `instanceof ApiError`; (3) three detail pages (`vendors/[id]`, `products/[id]`, `procurement/purchase-orders/[id]`) had their `err instanceof ApiError ? err.code : (err as {code?:string})?.code` conflict-detection simplified to the duck-typed form only. Also stripped the now-dangling commented-out "real implementation" blocks (`// import { apiRequest... } from "@/lib/api-client"` etc.) from all 15 `lib/api/*.ts` files, since they referenced a now-deleted module. `tsc --noEmit`/`eslint` clean (same pre-existing `login/page.tsx` error and established `set-state-in-effect` pattern only — confirmed unchanged by this pass).
+>
+> **If real backend integration is attempted again**, note two things found during the reverted pass: the RFQ backend contract had drifted from this file's assumed shapes (`Backend/ERPCore/Dtos/Procurement/RfqDtos.cs`/`RfqService.cs` — no persisted invited-vendor list, `requisitionId` required on create, different comparison DTO field names) — re-verify against the actual backend rather than trusting old assumptions; and a typed fetch client + ETag/error-normalization layer will need to be rebuilt from scratch since `lib/api-client.ts`/`lib/auth-token.ts` no longer exist.
+
 ## 1. Auth
 - [~] Login screen — UI built (`app/login`); not yet wired to `POST /auth/login` / token storage
 - [~] Forgot password — add email screen — UI built (`app/login/forgot`); not yet wired to API