Add Phased Angular Migration Plan: Version 9 to 19

[devin-ai-integration]

Add Phased Angular Migration Plan: Version 9 to 19

Summary

This PR adds a comprehensive migration plan document (ANGULAR_MIGRATION_PLAN.md) that provides a detailed roadmap for upgrading the Angular2-HN application from Angular 9.0.1 to Angular 19. The document includes:

• Current state assessment identifying 6 known issues in the codebase (deprecated RxJS imports, Zone.js import path, removed extractCss config, TSLint deprecation, legacy rxjs-compat package, service worker config)
• 10 sequential migration phases (9→10→11→12→13→14→15→16→17→18→19)
• Pre-migration preparation steps
• Detailed migration steps for each phase with commands, dependency versions, and code changes
• Verification checklists for each phase
• Testing strategy (unit, integration, performance, PWA)
• Rollback strategy
• Estimated timeline of 45-65 hours total effort

Review & Testing Checklist for Human

• Verify the current state assessment matches the actual codebase (check package.json, angular.json, and the identified deprecated imports in src/app/user/user.component.ts, src/app/shared/services/hackernews-api.service.ts, src/app/item-details/item-details.component.ts, and src/polyfills.ts)
• Confirm the migration phases are in the correct sequential order and no major versions are skipped
• Review the timeline estimates for reasonableness based on team experience with Angular migrations

Notes

This is a documentation-only PR with no code changes. The plan should be cross-referenced with the official Angular Update Guide at https://update.angular.io when executing each phase.

Link to Devin run: https://app.devin.ai/sessions/edc735a6899742aa9566c6acb92593d2
Requested by: Eddie Mattout

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