Fix: CI migration coverage, stale docs, ui-demo gaps, cn() duplication

[MethelyDev]

Summary

Four independent issues addressed in separate commits on this branch:

closes #1252
closes #1245
closes #1246
closes#1248

[CI] Add migration-from-scratch CI job (#1252)

The migration-check job already had a Postgres service and ran migrate:up, but was missing:

• A migrate:down → migrate:up cycle to catch irreversible migrations and ordering collisions (the duplicate timestamp prefixes noted in the issue would surface here)
• Path-based triggering so the job only fires on PRs that touch backend/migrations/** or backend/package.json, while always running on pushes to main

Changes:

• .github/workflows/ci.yml — add a migration-paths job using dorny/paths-filter@v3; make migration-check depend on it with an if guard; extend the job with a full down-loop then a second migrate:up run

[Docs] Rewrite stale frontend/README.md (#1245)

Every section of the README described planned features as if they hadn't shipped. Updated to reflect the actual codebase:

• Tech Stack — add Zustand, React Query, @stellar/freighter-api, next-intl, Sentry, Framer Motion, Recharts; remove never-integrated Stellar Wallet Kit
• Available Scripts — add test, test:watch, test:e2e, typecheck, format, audit:a11y; correct lint description (prettier --check, not ESLint)
• State Management — document the six Zustand stores instead of planned Context API
• Wallet Integration — document @stellar/freighter-api usage instead of the planned kit
• Component Library — document all 22 ui/ components with prop tables
• Routing — replace the stale / and /404 table with the full /[locale]/* route list
• Project Structure — remove non-existent tailwind.config.ts; document Tailwind 4's CSS-first @import "tailwindcss" + @theme token approach in globals.css
• Testing — document Jest unit tests and Playwright e2e specs as already shipped

Changes: frontend/README.md

[Docs] Extend ui-demo gallery and gate route from production (#1246)

The ui-demo route only showcased 4 of 22 components and silently shipped to production under every locale.

• Production gate — notFound() called at the top of the page when NODE_ENV === "production"; the route returns 404 in all production builds
• Extended coverage — gallery now covers 10 components: Button, Input, Card, Modal (existing) + Skeleton (all 4 variants), EmptyState, StatusIndicator, LoanStatusBadge, Tooltip, PaginationControls, CopyButton, ThemeToggle

Changes: frontend/src/app/[locale]/ui-demo/page.tsx

[Frontend] Remove duplicate cn() helpers in ui/ components (#1248)

Button.tsx, Card.tsx, and EmptyState.tsx each copy-pasted an identical local cn(clsx+twMerge) function instead of importing the shared helper already used by Input.tsx and Modal.tsx.

• Removed local cn function and the unused clsx / twMerge imports from all three files
• Added import { cn } from "@/app/utils/cn" to each

Changes: frontend/src/app/components/ui/Button.tsx, Card.tsx, EmptyState.tsx

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Test plan

• CI: open a PR that modifies a file in backend/migrations/ and verify the migration-check job runs; open one that does not and verify it is skipped
• CI: confirm migration-check passes the full up → down → up cycle on the 34 existing migrations
• Frontend: run npm run typecheck in frontend/ — should pass with no new errors
• Frontend: visit /en/ui-demo in development — should render all sections including Skeleton, EmptyState, etc.
• Frontend: build for production (npm run build) and confirm /en/ui-demo returns 404
• Frontend: run npm test — existing unit tests should still pass (cn() change is behaviour-preserving)
• Docs: verify README scripts section matches package.json scripts

Files changed

.github/workflows/ci.yml
frontend/README.md
frontend/src/app/[locale]/ui-demo/page.tsx
frontend/src/app/components/ui/Button.tsx
frontend/src/app/components/ui/Card.tsx
frontend/src/app/components/ui/EmptyState.tsx