fix(erd): cap PNG export size, make SVG export look like the screen #1153

Problem

• PNG export has no size cap (erd_view.dart:152): the whole canvas is rendered at pixelRatio: 2. A schema of 150–200 tables gives a canvas of several thousand px per side, i.e. an image over 10 000 px per side; this can exhaust memory or exceed the GPU texture limit.
• SVG does not look like the screen (erd_export.dart): positions and routes match, but there are no crow's foot / bar end markers, no rounded corners, PK / FK are [PK] text instead of icons, the header has no tint, and long types run past the card edge (no clipping).

Scope

• PNG: pick pixelRatio so that the longest side is at most 8192 px (fall back to 1 or below); if even that is too big, say so and offer SVG.
• SVG: the same markers (crow's foot at the FK end, bar at the referenced end), rounded corners (same roundedPath geometry), header tint, PK / FK glyphs, types clipped to the card (clipPath or truncation with …).
• The relation <title> stays.

Acceptance

• Unit test: the PNG ratio for a 20 000 × 9 000 canvas keeps the longest side ≤ 8192.
• Golden-style test on the SVG string: markers present for every relation, no text wider than its card.