feat(er-diagram): export the schema as Mermaid or DBML

[gcapellib]

What

The ER diagram had no export at all. This adds an Export button to the toolbar, opening a menu with two text formats generated from the schema data already in memory — no new dependency.

Why two formats

They cover different needs:

• Mermaid (erDiagram) renders natively on GitHub, GitLab, Notion and most docs tools, so the output can be pasted straight into a README. Its relationships are entity-level, so the FK column only appears as a label.
• DBML keeps relationships at column level (Ref: commandes.client_id > clients.id) and round-trips through dbdiagram.io and dbml-to-sql. Composite primary keys use an Indexes block, as inline [pk] cannot express them.

For reference, DBeaver's ER export is image/GraphML only. Its open request for Mermaid export (#36058) shows users falling back to exporting GraphML and hunting for converters that don't seem to exist.

Also fixed: permissions in the ER diagram window

src-tauri/capabilities/default.json allowed the literal window name "er-diagram", but the window is created as er-diagram:{connectionId}:{database}:{schema} (commands.rs:3874). Every permission-gated call was therefore rejected in that window:

dialog.save not allowed on window "er-diagram_..._boutique_"
allowed on: [..., "er-diagram", ...]

Changed to "er-diagram*", matching the wildcard already used by json-viewer-*, results-window-* and connection-window-*. This is a pre-existing bug independent of the feature — any plugin call needing a permission in that window was blocked.

Implementation

• Generators live in src/utils/schemaExport.ts, separate from the component so they can be unit-tested.
• Export uses allNodes/allEdges rather than the rendered set, so the file stays complete even while a single table is focused.
• The format menu reuses the existing ContextMenu component.
• Both generators de-duplicate edges: the diagram currently emits duplicate edges for some FKs (React logs Encountered two children with the same key), which would otherwise produce repeated relationships. Worth a separate look, but the export is defensive about it.

Testing

14 unit tests (Vitest) covering identifier sanitising, PK/FK markers, relationship direction, composite keys, de-duplication and empty tables.

Manually verified on MariaDB against a 15-table schema exercising self-references (employes.manager_id, categories.parent_id), two FKs from one table to the same target (commandes.adresse_livraison_id and adresse_facturation_id), three composite PKs and a table referenced by five others. Both outputs render correctly in mermaid.live and dbdiagram.io.

npm run typecheck clean.

Possible follow-up

Image export (PNG/SVG) would cover a different need — pasting the diagram into a slide or a document. React Flow documents a path for it but it needs html-to-image as a new dependency, so I left it out rather than decide that here. Happy to add it in a follow-up if you'd want it.