Canvas: connections between cards

[KrasForge]

Summary

Draw connections between note cards on the canvas and persist them alongside card positions.

Part of #342. Depends on the note cards sub-issue.

Behavior

• Draw a link between two cards by dragging from one card's edge to another.
• Remove a connection.
• Connections persist client-side with the rest of the canvas layout and redraw correctly through pan and zoom.

Implementation notes

• Reuse the edge-drawing approach from GraphView; links follow the same view {x,y,k} transform as the cards.
• Persist connections to localStorage in the same per-canvas record as card positions.
• Connections are canvas-local layout, independent of note-to-note links in the knowledge graph, unless a later issue chooses to bridge them.

Acceptance criteria

• A connection can be drawn between two cards and removed.
• Connections survive a reload and track cards as they are dragged.
• Renders correctly in light and dark themes.