feat: add landing page and app route

[vycdev2]

Summary

• Add a responsive, asset-backed landing page at / with workflow guidance and product/community links.
• Route the browser tool to /app, keep docs at /docs/..., bypass the landing page in Tauri, and preserve existing localStorage data.
• Add route-aware SEO, app noindex metadata, structured data, a stable /app manifest start URL, and static dist/app/index.html output.
• Move Docs, Discord, GitHub, and Support Me into the Settings Resources and community section, with keyboard-modal focus handling.
• Split the tool into a lazy-loaded app bundle and polish the landing page for desktop/mobile contrast, touch targets, keyboard focus, reduced motion, scrolling, and overflow.
• Verify generated root/app/docs/robots/sitemap/manifest/version artifacts during every build and preserve hashed documentation diagrams in static SEO pages.

Verification

• npm run build — passed; generated-output verification covered root, app, docs, robots, sitemap, manifest, version, canonical URLs, and documentation image assets.
• npm run lint — passed with zero warnings.
• npm test — passed (339 tests).
• PLAYWRIGHT_BROWSERS_PATH=/opt/data/.playwright npm run test:e2e — passed (6 smoke tests, including STL/3MF export flow and 5 landing/route/settings/persistence tests).
• Browser screenshots and computed-layout checks — passed at 1440×1000 and 375×812; no horizontal overflow and navigation targets are 44px high.
• /version.json, /app/, and a generated documentation diagram — served successfully from the production preview with the expected content types.
• git diff --check — passed.
• npm run tauri:build -- --no-bundle — frontend build passed; native compilation is blocked because this runner lacks the glib-2.0 >= 2.70 system development package.

Risk

• Medium: this changes web entry routing and static page generation, while the existing tool workflow remains mounted unchanged under /app.

Closes #62

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