Add creative guided canvas

[Flow-Fly]

Summary

• derive creative coverage from visible painted cells without grading color correctness
• render every remaining target number at 1600% and above, with a low-zoom hint below that threshold
• replace palette editing in guided projects with semantic numbered colors, remaining counts, progress, and selectable artist-added colors
• keep guide coordinates aligned by disabling structural canvas, palette, layer, frame, and reference-image changes while preserving drawing, arbitrary colors, navigation, undo/redo, export, and CRT
• remove the dormant duplicate layer toolbar uncovered during the live pass

Product behavior

Guided drawing remains a scaffold for artists. Any painted color covers a cell, off-guide colors remain valid, erasing reveals guidance again, progress is informational, and nothing in this slice decides when the work is complete.

Issues

Closes #311
Closes #312
Closes #313
Closes #314

Part of #130 and #128.

Verification

• npm run test:run — 476 tests passed
• npm run lint -- --quiet
• npm run build
• npx fallow audit --changed-since origin/develop — changed-scope gate passed
• git diff --check
• live browser pass at fitted 1109% and readable-number 1600% zoom, including numbered palette and fixed layer/frame controls

The production build retains the existing dynamic-import and large-chunk warnings.