fix(erd): a diagram with every table hidden says so, the empty state names the database (#1157)

[ZhuchkaTriplesix]

Closes #1157

What was wrong

The toolbar surface, the iconed actions, the Export ▾ menu, the database-based file names and the QueryaEmptyState / QueryaSpinner states were already in place. Still off:

• A false "No tables found". With every table hidden through "Hide from diagram", the view said "This schema has no tables, or this role cannot see them", although the tables exist. That state had no way to bring them back.
• The empty state did not say which schema was shown, which the issue asks for.
• Copy Mermaid was not covered by a test, while the acceptance asks for the Export menu items to be covered (Mermaid and SVG were; PNG came with fix(erd): cap PNG export size, make SVG export look like the screen #1153).

Changes

• "All tables are hidden" state with the count and a Show all tables action.
• "No tables found" names the database: "The current schema of <db> has no tables, or this role cannot see them."
• Tests: Copy Mermaid puts the diagram on the clipboard; hiding every table shows the new state and Show all tables brings them back; the empty state names the database.