fix(structure): fit nanoscroll view after build

[XCai-0213]

Summary

• Center generated nanoscroll atoms inside the non-periodic cell so the lattice box no longer encloses an off-center scroll.
• Refit the viewer after nanoscroll construction by resetting the camera position and orbit target.
• Recompute orthographic zoom when the structure size changes, so newly generated large nanoscrolls are visible without manual zooming out.

Verification

• cargo test nanoscroll --lib passed: 5 tests passed.
• pnpm build:wasm passed.
• git diff --check passed.
• pnpm test was attempted but timed out after 120s with a Vitest output pipe EPIPE.
• pnpm check was attempted; it still reports existing project-wide diagnostics: 11 errors and 303 warnings.