feat(erd): a new group gathers its tables when its frame would cover other cards

[ZhuchkaTriplesix]

Follow-up to #1282, from the interface review (item 3), decided: gather the group's tables.

Problem: marking tables far apart and grouping them drew a frame over every card between them. Only Auto layout kept a group together, and it also drops every manual position.

Now: when the frame of a new group — or of a group a table was just added to — would cover a card that is not a member, the group's tables are packed into one block (ErdLayout.gather):

• members keep their reading order (top to bottom, left to right) in a near-square grid with room for the edges between them;
• the block goes at the first member's place, else at the next member's, else to the right of every other card, wherever its frame covers no other card;
• cards that are not members never move, and a toast says that the tables were moved together;
• attached notes follow their tables;
• when the frame is already clear nothing moves, so an arrangement the user made is not reshuffled.

Tests: ErdLayout.frameCoversOthers / gather in test/core/erd/erd_groups_test.dart (detection, block clear of others and others unchanged, far-right fallback, no-op cases) and a widget test in test/features/erd/erd_groups_view_test.dart (users + tags around orders: orders keeps its place, the two end up together, the toast shows).