refactor: environment-specific config, lazy route loading, payment strategy pattern, component-based email templates

[CeceOs92]

Summary

Implements four platform refactors: environment-aware configuration with secret management, route-level code splitting for the frontend, a strategy pattern for multi-chain payments, and a component-based email templating system.

Closes #606
Closes #607
Closes #608
Closes #609

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#606 — Environment-specific configuration

• Per-environment override files (backend/src/config/environments/{development,staging,production}.ts), merged into process.env before Zod validation in config.ts
• Optional AWS Secrets Manager integration (lazy-loaded, opt-in via AWS_SECRETS_MANAGER_ENABLED) with a refreshConfig() runtime-refresh capability
• npm run config:validate / config:drift / config:docs scripts + .github/workflows/config-validation.yml, auto-generated docs/config/CONFIGURATION.md
• infra/main.tf: new aws_secretsmanager_secret.app_secrets + read policy per environment

#607 — Lazy-loaded, code-split frontend routing

• Route-level error boundaries: app/[locale]/error.tsx, app/admin/error.tsx, app/global-error.tsx
• Offline fallback page (app/offline), wired into public/sw.js as the final navigation fallback
• components/navigation/PrefetchLink.tsx — hover/focus prefetching via router.prefetch()
• components/analytics/RouteTransitionMetrics.tsx — route-transition timing reported through the existing web-vitals beacon
• Extended next.config.ts splitChunks to cover all feature routes (admin/payments/security/onboarding/logs/accessibility) and fixed the existing dashboard/auth/forms cache groups, which weren't matching the actual app/[locale]/... paths
• Bundle size budgets: bundle-budget.json, scripts/check-bundle-size.mjs, .github/workflows/bundle-size.yml

#608 — Strategy pattern for multi-chain payments

• Wired the existing (previously unused) PaymentProvider strategy interface, Stellar/EVM/fiat/credit strategies, and router into real traffic for the first time
• providers/mock.ts for deterministic strategy tests; payment-router.test.ts covers selection, fallback, and failure paths
• Token-aware strategy selection (PaymentInput.token)
• unified-payment-tracker.ts — persists every payment, regardless of strategy, into the existing Payment table
• New routes/payment-strategies.ts (/api/v1/payment-strategies) and a StrategyConfigPanel tab on /admin/configuration showing per-strategy health and metrics

#609 — Component-based email templates

• Handlebars partials for shared components (templates/components/{layout,header,footer,button}.hbs), auto-registered by EmailTemplateEngine
• renderWithLayout() implements template inheritance: content renders standalone, then injects into the shared layout
• templates/registry.ts — Zod-typed template definitions (renderTypedTemplate<K>) for compile- and runtime-safe variables, plus deterministic A/B variant bucketing
• Preview endpoints (GET /api/v2/email/templates/registry[/:id/preview]) and a preview UI at frontend/app/dashboard/emails
• Template versioning and delivery analytics were already implemented pre-PR and are unchanged

Scope notes

• Refactor configuration management to use environment-specific configs #606: no full npm install verification (workspace install was too slow in this environment) — scripts were validated in an isolated sandbox instead
• Refactor frontend routing to use lazy loading with code splitting #607: per-component dynamic() audit inside pages not done; route-transition metric is an approximation, not true click-to-paint
• Refactor payment processing to use strategy pattern for multi-chain support #608: contracts/src/lib.rs untouched (out of scope for a service-layer refactor); recommend running npm run test -w backend before merge
• Refactor email templates to use component-based templating engine #609: only welcome-email and payment-received migrated to the new registry as a proof of pattern; full template catalog migration and A/B-variant wiring into delivery analytics are follow-ups

Test plan

• npm run test -w backend (payment-router strategy tests)
• npm run config:validate && npm run config:drift in backend/
• npm run build && npm run bundle:check in frontend/
• Manually load /admin/configuration (Payment Strategies tab) and /dashboard/emails preview page

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