Migrate Tube Sheet Generator to a standalone Vite micro-app

[YurMil]

Execute the migration plan in dev-plans/tube-sheet-generator-microarchitecture-migration.md: move src/components/tools/TubeSheetGenerator into a Vite app under apps/tube-sheet-generator, published at static/utility-apps/tube-sheet-generator/app.html.

Why: The tool pulls Replicad/OpenCascade (WASM) into the main bundle today; isolating it improves site-wide load performance and build times.

Acceptance criteria:

• Feature parity: hole pattern layout, DXF and STEP export
• Replicad/OpenCascade loaded only inside the micro-app
• Docusaurus page uses the shared utility shell

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[YurMil]

Audit result: complete. Migrated to the standalone YurMil/Tube-Sheet-Generator- repo (Vite app deployed to static/utility-apps/tube-sheet-generator/ by CI), page served through createUtilityPage.

Verified against acceptance criteria:

• ✅ Feature parity — hole pattern layout, DXF and STEP export live (DXF format additionally fixed for strict viewers in Fix DXF export rejected by eDrawings/AutoCAD viewers Tube-Sheet-Generator-#13)
• ✅ Replicad/OpenCascade loads only inside the micro-app — zero replicad imports remain in the site's src/ (webpack only bundles imported modules)
• ✅ Docusaurus page uses the shared utility shell

Follow-up noted for #99 (bundle audit): the site's package.json still lists now-unused replicad/opencascade dependencies and the webpack async-WASM plugin config — removable to slim installs and CI.