feat(viewer): add Vue/Vite preview frontend

[wbugitlab1]

Refs #148

Summary

• Adds a hidden Vue 3/Vite preview frontend under src/viewer/app, built as a self-contained dist/viewer-next/index.html artifact.
• Keeps the legacy production viewer path, server routing, proxy behavior, and CSP contract unchanged.
• Adds typed viewer boundaries for route handling, runtime environment reads, API access, dashboard data, and preview build validation.

Verification

• corepack pnpm run viewer:test
• corepack pnpm run viewer:typecheck
• corepack pnpm run build
• corepack pnpm run lint
• corepack pnpm exec vitest run test/viewer-security.test.ts test/viewer-host.test.ts test/viewer-server-routing.test.ts test/viewer-i18n.test.ts test/build-package-contract.test.ts
• corepack pnpm test
• Browser smoke: loaded dist/viewer-next/index.html?smoke=5#dashboard through a local HTTP server with 0 console errors/warnings
• osv-scanner scan source .
• semgrep scan --config p/default --error --metrics=off .
• gitleaks protect --staged --redact

Notes

• The Vue/Vite frontend is preview-only in this PR; no production viewer cutover is included.
• No Playwright/browser dependency was added. Browser smoke coverage was performed manually for the preview artifact.