[Feature] Editable vertex in shapes and polygon recognition

[marcocebrian]

Summary

Make shapes first-class, editable geometry, not just bounding-box primitives.

1. Every recognized or drawn polygon/shape should expose editable vertices (drag, add, delete).
2. Freehand strokes should support polygon / shape recognition (triangle, rectangle, diamond, ellipse, n-gon, closed irregular polygon).
3. Auto-detected closed shapes must be fillable, including semi-transparent color (alpha independent of stroke).

This should work on the same canvas as notes and tables, so a table, a labeled polygon, and a filled region can live on one page.

Proposed behavior

1. Editable vertices

When a closed or open path/shape is selected:

• Show vertex handles at every point.
• Drag a vertex to move it.
• Click on an edge (or double-click) to insert a vertex.
• Delete / Alt-click / backspace a selected vertex to remove it (min 3 points for a filled polygon).
• Optional: mid-edge “add point” handles, like vector editors.
• Snapping: vertex-to-vertex, vertex-to-grid, vertex-to-table cell edges / guides if tables exist on the canvas.
• Undo/redo each vertex mutation as one history step.

2. Polygon / shape recognition

On stroke end (or via an explicit “Recognize” action / toggle):

Detect, in order of preference:

• line
• rectangle / rounded rect / square
• ellipse / circle
• triangle, diamond, regular n-gon (5–8)
• closed irregular polygon (simplified polyline, Ramer–Douglas–Peucker or similar)
• arrow (optional)

UX:

• Hold a modifier (e.g. Shift) or use a “Snap to shape” toggle so recognition is opt-in, not forced.
• After snap, keep the recognized geometry as a path with vertices, not a locked primitive — user can immediately pull a corner.
• Confidence threshold: if the stroke is not close enough, leave ink as-is.
• Recognize both freehand ink and multi-segment polylines.

3. Fill + transparency on auto-detected shapes

Any closed recognized path must support:

• Fill color (palette + custom hex)
• Fill opacity 0–100%, separate from stroke opacity if possible
• Stroke color / width unchanged when fill changes
• Fill rule: even-odd for self-intersecting polygons
• Tables / text under the shape remain readable at ~15–40% fill (this is the main use case)