Replace PNG/SVG upload with an Image import wizard (5+ image-to-lines algorithms)

[BernardJen]

Summary

Replace the current separate upload PNG / upload SVG actions with a single "Upload image" entry that opens an import wizard. The wizard lets the user adjust the image and choose how it is converted to plottable lines, with at least 5 conversion algorithms.

Motivation

The current flow exposes two distinct upload paths and limited conversion control. A unified wizard makes import discoverable and gives the user control over the line-conversion result before committing to a plot.

Wizard flow

1. Upload image (raster or vector).
2. Adjust the image — basic edits such as: crop, scale/fit, rotate, brightness/contrast, threshold, invert.
3. Choose conversion algorithm with a live preview of the resulting lines.
4. Tune algorithm parameters (e.g. line spacing, threshold, density).
5. Confirm — paths are added to the canvas.

Algorithms (at least 5)

Suggested set (final list TBD during design):

1. Edge detection / outline tracing
2. Horizontal/vertical/diagonal hatching by brightness
3. Crosshatch (multi-angle hatching by tone)
4. Dithering / stippling (dots)
5. Contour / iso-level lines
6. Centerline / skeleton tracing (bonus)

Acceptance criteria

• A single "Upload image" action replaces the separate PNG and SVG uploads.
• The wizard supports basic image adjustments before conversion.
• At least 5 selectable image-to-lines algorithms are available.
• Each algorithm shows a live preview and exposes its key parameters.
• Confirming the wizard adds the generated paths to the canvas.

Notes

Current conversion logic lives under src/plot/ (raster.ts, svg.ts, detail.ts); reuse/extend where possible.

[BernardJen]

Superseded by split issues #11 (wizard shell + adjust + 2 algorithms) and #12 (additional algorithms to reach 5+). Keeping this open as the umbrella, or close in favor of #11/#12. Tracked in epic #10.