feat(erd): the diagram keeps its layout: positions, collapsed and hidden tables, detail level, viewport #1275

Part of #1286.

Problem

Positions of the cards, collapsed and hidden tables are lost when the Diagram tab (or the app) closes. Every open starts from the computed layout, so arranging a diagram is wasted work. This is the main reason the diagram feels unfinished next to dbdiagram, where the layout is part of the document.

Scope

• Store per connection + database + schema: card positions, collapsed tables, hidden tables, detail level, zoom and scroll offset.
• A new LocalDb table (erd_layouts: connection_id, database, schema, JSON payload, updated_at), removed with its connection.
• Save on drag end, collapse, hide / show and detail-level change (debounced); restore on open.
• Tables that appeared since the layout was saved are placed by the layout engine next to their neighbours, without moving the saved ones; tables that no longer exist are dropped from the saved layout.
• Auto layout keeps working and replaces the saved positions (one undoable step is enough: Restore saved layout until the next change).

Acceptance

• Arrange, close the tab, reopen: same positions, collapsed / hidden tables, detail level and viewport.
• A new table in the database is placed without moving the arranged ones.
• Removing the connection removes its layouts.
• Tests: save / restore round trip, new and dropped tables, migration.