fix(image): guard raster dimensions

[ubugeeei]

Summary

• use checked arithmetic for decode buffer sizes
• return structured errors for zero scale and oversized render dimensions
• propagate render validation through the WASM canvas APIs

Closes #2
Closes #3

Validation

• cargo test -p fastqr-image
• cargo clippy -p fastqr-image -p fastqr-wasm --all-targets --target wasm32-unknown-unknown -- -D warnings

[ubugeeei]

Landed via #27 after local vp run ci and final GitHub Actions CI passed. Closing this issue-scoped PR to keep the queue and PR list clean.