chore: set up OpenSpec spec-driven workflow

Add openspec/config.yaml with project context (React 19/Vite, FSD layers,
auth flow, CSS Modules) and per-artifact rules so agents generate proposals,
specs, designs, and tasks aligned with project conventions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

openspec/config.yaml

@@ -0,0 +1,51 @@
+schema: spec-driven
+
+# Project context (optional)
+# This is shown to AI when creating artifacts.
+# Add your tech stack, conventions, style guides, domain knowledge, etc.
+context: |
+  Project: elcsa frontend — a crypto/finance web app (currency converter,
+  wallet, networks, auth) for the ЭКСА platform.
+
+  Tech stack:
+  - React 19 + Vite, TypeScript (strict, tsc -b before build)
+  - TanStack Query (React Query) for server state
+  - React Router for routing
+  - Plain CSS Modules (.module.css) — NO Tailwind, NO CSS-in-JS
+  - Design tokens live in src/app/styles/variables.css
+
+  Architecture — Feature-Sliced Design (FSD). Layers are strictly ordered;
+  a lower layer must never import from a higher one:
+    app → pages → widgets → features → entities → shared
+  Path aliases map to layers: @app/* @pages/* @widgets/* @features/*
+  @entities/* @shared/*. Each widget/feature owns its model/, ui/ and a
+  barrel index.ts.
+
+  Key conventions:
+  - Auth is two-step email+code for both register and login. Access token is
+    kept in an in-memory tokenStore (NOT localStorage); refreshed via an
+    HttpOnly cookie against the auth service. Every request attaches a cached
+    CSRF token and retries once on 401 after a silent refresh.
+  - Reusable UI lives in shared/ui (Button, PrimaryButton, FormField, Pill,
+    Title, TokenIcon, Notification), each with its own CSS Module.
+  - Routes are centralized in shared/config/routes.ts (ROUTES const).
+    ProtectedRoute / GuestRoute guard auth-only and guest-only pages.
+  - VITE_API_URL must be set (src/shared/config/env.ts).
+  - No test runner is configured.
+
+# Per-artifact rules (optional)
+# Add custom rules for specific artifacts.
+rules:
+  proposal:
+    - Keep it concise and focused on user-facing behavior and the "why".
+    - State which FSD layer(s) the change touches and any cross-layer impact.
+  specs:
+    - Write requirements as observable behavior, not implementation details.
+    - Cover the error/edge paths (network failure, 401 refresh, validation).
+  design:
+    - Place new code in the correct FSD layer and justify the placement.
+    - Reuse existing shared/ui components and shared/api helpers before adding new ones.
+    - Note any new env vars, routes (ROUTES const), or React Query keys.
+  tasks:
+    - Break work into small, verifiable steps ordered by FSD dependency (shared → up).
+    - Include a "type-check passes (npm run build)" and "npm run lint" step; there are no automated tests.