fix: ensure components compile under noUncheckedIndexedAccess

[tobigumo]

Summary

• Enable noUncheckedIndexedAccess in the components template tsconfig so downstream CLI consumers with strict tsconfig no longer hit type errors on the shipped templates
• Fix file-upload FileText in both react and solid: the length === 1 guard did not narrow indexed access; replaced with destructuring + truthy check so neither ?. nor ! is required
• Fix solid checkbox/indeterminate example by capturing the indexed value locally so the truthy guard narrows for the subsequent spread
• Regenerate the file-upload registry output to match the updated source

Test plan

• bun run typecheck passes in components/react and components/solid
• Registry output for file-upload regenerated; other registry JSONs left untouched