ui(erd): toolbar, Export menu and empty/error states per UI kit #1157

Problem

The Diagram tab does not follow the UI kit (docs/ui-kit.md):

• The toolbar is a bare Wrap with 8 px padding (erd_view.dart:230): no surface, no bottom border, unlike every other toolbar.
• Mermaid / SVG / PNG are three separate buttons without icons.
• Error state is a raw Text(_error!) with no retry (:167); empty state is a raw Text('No tables found') (:170); loading is a spinner without a label.

Scope

• Toolbar with the standard toolbar decoration (SqlEditorChrome.sqlToolbarDecoration or the shared table toolbar surface), actions as QueryaActionButton with icons.
• One Export ▾ (QueryaActionMenu): Mermaid, SVG, PNG, Copy Mermaid to clipboard. File names from the database (<db>-erd.svg), not diagram.svg.
• QueryaEmptyState for error (with Retry) and for "no tables" (with a hint about the schema shown); QueryaSpinner with Loading schema….
• The Relations view in the table browser (feat(erd): Relations view in the table browser #1149) reuses the same pieces.

Acceptance

• No raw Text states and no unlabelled export buttons in erd_view.dart.
• Export menu items covered by widget tests (existing keys erd_mermaid, erd_svg, erd_png kept on the menu items or tests updated).