fix(a11y): give the graph and its controls accessible names

[Ravi chandu Edru (RaviChanduEdru)]

Picks up the unchecked "Add ARIA labels to the graph visualization" item from TODO.md, continuing the recent accessibility work (#75, #77, #78).

The problem

Cytoscape paints the ontology into <canvas> elements, which expose nothing to assistive technology. The container is a bare div:

<div ref={containerRef} className="graph-canvas" data-testid="ontology-graph-canvas" />

No role, no label, no text alternative — a screen reader announces nothing at all for the app's central feature.

The change

The graph canvas gets role="img" and an accessible name built from the loaded ontology — name, entity-type count, relationship count:

"Fourth Coffee ontology graph: 6 entity types, 7 relationships. Interactive diagram."

Two deliberate choices there:

• The role goes on .graph-canvas, not .graph-container. role="img" prunes descendants from the accessibility tree, and the container also holds the legend and the control buttons — putting it there would have hidden them.
• Entity and relationship names are not folded into the label. They're already reachable as real DOM in the search panel and the legend, and a paragraph-long accessible name reads badly.

The summary lives in src/lib/graphA11y.ts as a pure function, so it's testable without mounting cytoscape in jsdom.

The five control buttons are icon-only and carried only a title. To be precise: title does supply an accessible name for an icon-only button, so this wasn't a violation — but it's the fragile option, announced inconsistently across screen readers and invisible on touch. They now carry explicit aria-labels, with title kept for the mouse tooltip.

Verification

5 unit tests cover the summary: counts, singular/plural agreement, an empty ontology, and the unnamed/whitespace-name fallback.

Driven against the running app with Playwright:

Check	Result
Canvas exposes role="img"	yes, 1 match
Accessible name	"Fourth Coffee ontology graph: 6 entity types, 7 relationships. Interactive diagram."
Label tracks the loaded ontology	switches to "E-Commerce Platform ontology graph: 5 entity types, 6 relationships."
All 5 controls resolvable by accessible name	yes — and they activate correctly when clicked by name

No console errors. Only existing theme tokens are involved, so the contrast suite is untouched.

Scope

• Keyboard navigation of the graph is a separate unchecked TODO item and is deliberately not addressed here — it's a design question, not a labelling one.
• Independent of fix(lint): repair no-case-declarations errors and gate lint in CI #108 and fix(graph): make the entity type legend collapsible #109. fix(graph): make the entity type legend collapsible #109 also touches OntologyGraph.tsx, but a different region (the legend), so they merge cleanly in either order.
• npm test on this branch shows one unrelated pre-existing failure — compile-catalogue.test.ts timing out under Vitest's 5s default — which is what fix(lint): repair no-case-declarations errors and gate lint in CI #108 fixes.