fix(erd): clean PNG capture, SVG export in the theme's colours like the screen (#1153)

[ZhuchkaTriplesix]

Closes #1153

What was wrong

• PNG captured the interaction state. The RepaintBoundary holds the whole canvas, so a picked table (other cards at 35 % opacity), the hover highlight and the edge tooltip ended up in the image.
• PNG failures were silent. _exportPng() was not awaited and had no error handling; a canvas too big for the GPU failed without a message.
• SVG did not look like the screen. Fixed light colours (in a dark theme the PNG was dark and the SVG light), card radius 6 instead of 8, sans-serif types instead of monospace, column names shifted by the PK / FK pills instead of lining up, no column count in the header.
• The acceptance test was weak. It checked one relation's <title>, not markers for every relation, and only long types were checked for fitting, not table or column names.

Changes

• PNG: the picked table, hover highlight and edge label are cleared for one frame before the capture; the pick is restored afterwards. Errors show a toast that points to SVG.
• SVG: ErdSvgColors (light defaults); the diagram view passes the current theme's colours. Card radius 8, header tint and column count, one key slot per card, monospace types.
• Edges and end markers carry erd-edge / erd-ends classes.
• Tests: an edge and end markers for every relation; long table and column names shortened; names of a card line up; caller colours are used; PNG export works while a table is picked.