refactor(erd): ErdSource per connection, schema-aware catalog, neighbourhood load

[ZhuchkaTriplesix]

Problem

The ER diagram (#1063, #1145) works only as a tab of the SQL workspace, and its data layer does not fit the table browser:

• ErdView takes a SqlExecutionDelegate, i.e. the SQL editor's session. The table browser has its own session on purpose (fix(postgres): do not share the SQL-editor read-write socket with Table Browser #785, fix(sqlite): do not share the SQL-editor Database with Table Browser #794, fix(mysql): do not share the SQL-editor socket with Table Browser #802) and must not borrow the editor's.
• ErdCatalog reads only current_schema() (PostgreSQL) / DATABASE() (MySQL). A table in another schema, or a foreign key into another schema, is not shown. ErdTable names are unqualified.
• The whole catalog is loaded every time. For one table only its neighbours are needed.
• GenericSqlWorkspace shows Diagram for extension drivers too. ExtensionSqlWorkspace passes SqlDialect.mysql, so the diagram runs MySQL catalog queries against whatever the extension connects to (e.g. ClickHouse).

Scope

• ErdSource: loadSchema({String? schema}) and loadNeighbourhood({required ErdTableRef table, int depth = 1}). Implementations for PostgreSQL, MySQL, SQLite on top of a query runner: the SQL workspace delegate for the Diagram tab, the table browser's own read-only session for the table view.
• Schema-aware catalog: the schema / database is passed explicitly. ErdTable gets schema; a table outside the current schema is labelled schema.table (cards, Mermaid, SVG).
• Neighbourhood: the foreign-key query filtered on the table (either side), repeated up to depth; then columns only for the tables found. Cycles and self references terminate.
• Availability: the workspace knows whether a diagram source exists; extension SQL workspaces get no Diagram button until an extension can declare one.
• ErdView takes an ErdSource instead of the delegate + dialect, plus an optional focusTable (accented card, viewport centred on it).

Acceptance

• The Diagram tab behaves as today; test/features/erd/erd_test.dart passes (updated only for the schema qualifier).
• PostgreSQL: a table in schema sales with a FK to public.customers: the neighbourhood contains both, qualified.
• Unit tests for the neighbourhood with a fake runner: depth 1 and 2, a cycle, a self reference, a table without relations.
• The extension SQL workspace has no Diagram button.