feat(project-editor): centralize schema-driven validation

Milestone 2 of the Configuration Engine sprint.

- Add schema/validators/primitives: pure isValidHexColor/HttpUrl/Email,
  validateJson, validateCss, extractStyleBlocks, normalizeRoute. One function
  per concern, no duplicated validator logic.
- Refactor ProjectValidator to compose the primitives and tag every issue with
  section + fieldKey + severity ('error' blocks publish, 'warning' advisory).
  Preserves all existing codes/messages; adds duplicate-routes, invalid-css,
  invalid-widget-config checks.
- Facade: issuesByField, issuesBySection, blockingIssues, hasBlockingIssues;
  publish() now gates on severity==='error' instead of any issue.
- i18n: add validationInvalidJson/Css/DuplicateRoutes/InvalidWidgetConfig to
  the Translations interface + en/ru/hy.
- Specs: primitives + ProjectValidator (25 passing total).

Gate: tsc --noEmit, npm test (25/25), arch:check, build all green.

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

src/app/i18n/en.ts

@@ -531,6 +531,10 @@ export const en: Translations = {
     validationInvalidColors: 'One or more theme colors are not valid hex colors.',
     validationMissingTranslations: 'Some content is missing translations for a supported language.',
     validationInvalidLayouts: 'One or more layouts reference an unknown layout type.',
+    validationInvalidJson: 'The configuration is not valid JSON.',
+    validationInvalidCss: 'A static page contains invalid CSS.',
+    validationDuplicateRoutes: 'Two or more pages share the same route.',
+    validationInvalidWidgetConfig: 'A widget is missing a required field (id, type, version, or props).',
     statusDraft: 'Draft',
     statusPublished: 'Published',
     unsavedChanges: 'Unsaved changes',