Implement phased modernization baseline: contract hardening, cart/order flow fixes, and documentation inventory

[Copilot]

This PR executes the missing implementable parts of the multi-phase modernization plan: locking a concrete frontend baseline, hardening frontend↔backend contract handling, and fixing critical cart/order flow issues that were still inconsistent with expected behavior.

• Baseline + scope artifacts

  • Added explicit baseline snapshot with current runtime/dependency/tooling state and known issues:
    • docs/baseline-snapshot.md
  • Added full frontend surface inventory (routes, components, services, directives, models, styles, assets):
    • docs/frontend-inventory.md
  • Added tracked mismatch register for contract gaps and remediation state:
    • docs/contract-mismatch-register.md
  • Updated documentation index and contract/testing docs to reflect current reality:
    • README.md
    • docs/api-contract-matrix.md
    • docs/testing-strategy.md

• Build/tooling stabilization

  • Added missing production environment replacement target:
    • src/environments/environment.prod.ts
  • Updated Angular production optimization config to avoid brittle external font inlining in constrained/offline environments:
    • angular.json

• Contract alignment + runtime guardrails

  • Hardened buy payload normalization to handle partial/legacy responses consistently (itemId + legacy itemid, safe defaults):
    • src/app/services/buy.service.ts
  • Refactored cart state mutation to deterministic ID-based behavior for add/remove/delete-all:
    • src/app/services/client.service.ts

• Critical cart/order flow remediation

  • Order submission now includes only user-selected cart items (not entire cart unconditionally).
  • Selection tracking now keys by buy.id instead of index, preventing desync after removals/reordering.
  • Cart checkbox state is synchronized after destructive operations.
  • Replaced alert/debug-driven UX in key paths with component status messaging:
    • src/app/components/cart.component.ts
    • src/app/components/htmls/cart.component.html

• Lifecycle and stability cleanup

  • Added explicit subscription lifecycle cleanup and safer document access in root component:
    • src/app/components/app.component.ts
  • Improved cleanup/status handling in creation flow:
    • src/app/components/creating.component.ts
    • src/app/components/htmls/creating.component.html
  • Removed residual debug-only console noise in directives/components:
    • src/app/components/buys.component.ts
    • src/app/components/htmls/buys.component.html
    • src/app/directives/check-page.directive.ts
    • src/app/directives/scroll.directive.ts

• Targeted test expansion (valuable logic)

  • Added coverage for cart selection semantics and checkbox sync after item removal:
    • src/app/components/cart.component.spec.ts
  • Extended service tests for defensive backend payload handling and ID-based full-remove behavior:
    • src/app/services/buy.service.spec.ts
    • src/app/services/client.service.spec.ts

Example of the key behavioral change (selection-safe order payload composition):

private getSelectedBuys(): Buy[] {
  const selectedById = new Map((this.checks.subchecks ?? []).map((check) => [check.name, check.set]));
  return Array.from(this.buysfororder).filter((buy) => selectedById.get(buy.id) ?? true);
}