fix(erd): edge label readable at any zoom, hover tolerance in screen px, Mermaid FK ends (#1156)

[ZhuchkaTriplesix]

Closes #1156

What was wrong

Nullability loading, the card's type? marker and the circle / bar edge ends (screen and SVG) were already in place. Still broken:

• The edge label never showed. The edge CustomPaint sat above the hover layer and is hit everywhere by default, so the hover layer never saw the pointer. The hover existed in code but did nothing in the app.
• The edge label scaled with the canvas. It was drawn inside the zoomed canvas: at the fit zoom of a large schema (now below 20 %) the 11 px text became unreadable, and at 300 % it was three times its size.
• The hover tolerance was in canvas units. 6 canvas px is under 1 screen px when zoomed out (an edge is nearly impossible to hit) and 18 px at 300 %.
• Mermaid ignored nullability. Every relation was ||--o{ (zero or many), so the export did not tell a nullable FK from a NOT NULL one, while the screen and the SVG did.
• No widget test for the edge hover, which the acceptance asks for.

Changes

• The edge painter takes no pointer (hitTest returns false), so hovering an edge reaches the hover layer.
• The edge label is drawn above the canvas in screen space, at the pointer's transformed position. It follows pans and zooms and is never in a PNG.
• Hover tolerance: 6 screen px at any zoom.
• Mermaid: ||--|{ for a NOT NULL foreign key, ||--o{ for a nullable one. This matches the circle at the FK end on screen, as the issue specifies.
• Tests: hovering an edge shows orders.user_id → users.id, and after zooming out the label keeps its screen size. Mermaid is checked for both nullable and NOT NULL.