feat: developer tooling - bundle size, coverage, dep automation, dev env

[Marvy247]

Summary

Implements developer tooling requested in issues #762, #763, #764 and #765.

• Implement bundle size monitoring with regression alerts #762 Bundle size monitoring with regression alerts

  • .size-limit.json: size budgets for framework/app chunks and CSS.
  • .github/workflows/size-limit.yml: builds the frontend and posts size regression comments on PRs via size-limit-action, plus enforces existing bundle budgets.
  • frontend/next.config.ts: emits size-limit-stats.json when SIZE_LIMIT=true for size analysis.

• Implement code coverage reporting with threshold enforcement #763 Code coverage reporting with threshold enforcement

  • frontend/vitest.config.ts: enabled lcov reporter, reports dir, exclusions and 80% thresholds (lines/branches/functions/statements).
  • .github/workflows/coverage.yml: runs coverage and uploads to Codecov.
  • codecov.yml: Codecov target/patch thresholds and PR comment config.

• Implement dependency update automation with Renovate/Dependabot #764 Dependency update automation (renovate/dependabot)

  • renovate.json: Renovate configuration with weekly schedule, grouping and lockfile maintenance.
  • .github/dependabot.yml: added workers, sdks and github-actions ecosystems.

• Implement local development environment with hot reload and orchestration #765 Local development environment with hot reload and orchestration

  • docker-compose.yml: added backend and frontend dev services with volume mounts + develop.watch for hot reload.
  • scripts/dev.sh: orchestration helper (up/down/logs/ps/restart/seed/watch).

Test plan

Developer tooling only; no runtime code paths changed. CI workflows validate the configuration on PRs.

Closes #762
Closes #763
Closes #764
Closes #765

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