Editor: toggles to show stitch connecting lines & origin links

[Eiluviann]

Placeholder — do not implement yet. Details to follow from the product owner.

Two view toggles in the editor:

1. Connecting lines between stitches — show lines connecting stitches (e.g. neighbouring stitch heads within a round), so the worked path reads at a glance.
2. Connecting stitches to their origin — show a line from each stitch to the stitch it was worked out of (its origin).

Notes for later specification:

• Both are display-only toggles; they don't change stored data. The selection overlay already draws an origin link for selected stitches (link() in src/core/editorCanvas.ts) — these toggles would show the relationships for the whole chart, not just the selection.
• Keep it uncluttered: faded/dotted styling, and likely off by default.
• State is a local UI preference (like the "How it works" collapse / Editor: collapsible "How it works" panel with remembered state #41), not part of the portable project format.
• Closely related to Configurable stitch-relationship presentation #53 (configurable stitch-relationship presentation) — decide whether this lands as the concrete first slice of Configurable stitch-relationship presentation #53 or as a sibling, and de-duplicate before implementing.