Polish guided labels and numbered-region bucket fill

[Flow-Fly]

Summary

• replace the dominant full-cell number badges with smaller medium-weight labels and a restrained contrast halo
• keep the existing 1600% threshold, number toggle, target preview, and painted-cell behavior
• make Bucket follow the four-connected guide region sharing the clicked number
• fill only transparent cells while preserving existing artwork
• use the artist's selected color rather than automatically solving with the target color
• leave normal-project bucket behavior unchanged

Product behavior

The bucket is a region assist, not an automatic solver. Separate islands and diagonal-only neighbors remain separate. Existing painted cells are preserved but do not split the underlying guide region.

Verification

• npm run test:run (89 files, 492 tests)
• npm run lint -- --quiet
• npm run build
• npx fallow audit --changed-since origin/develop
• git diff --check origin/develop...HEAD
• live visual check at the 1600% number threshold, with no new console errors

Closes #323
Closes #324