fix: accept single-pet folder zip uploads

[henryjing96]

Summary

• Support ZIP uploads where pet.json and spritesheet.webp/png are inside a single top-level pet folder.
• Prefer root-level pet files when both root and nested pet structures exist.
• Ignore macOS/system metadata files during ZIP classification.

Validation

• bun test src/lib/pet-zip.test.ts
• bun run check
• TELEMETRY_RATELIMIT_SECRET=mock-build-secret bun --env-file=.env.mock run build

[github-actions]

[important] src/lib/pet-zip.ts:65-71 misclassifies multi-pet archives when only one nested pet has a spritesheet. For example, boba/pet.json, boba/spritesheet.webp, and kebo/pet.json returns kind: "pet" because completeNestedCandidates.length === 1 runs before the later candidates.length > 1 bundle check. The submit form then normalizes and uploads only boba, silently ignoring the extra pet.json. That can turn an incomplete all-pets bundle into a single-pet submission instead of showing the bundle/error path. Consider checking for multiple non-root pet.json candidates before accepting the single complete nested candidate, and add a regression test for this partial-bundle case.

[henryjing96]

no need to support this feature