fix(graph): make the entity type legend collapsible

[Ravi chandu Edru (RaviChanduEdru)]

Fixes the first item of #87 — "a popup or overlay appears over the graph and obscures the nodes. I could not find an option to close, minimize, or move it."

The problem

The entity-type legend is absolutely positioned over the graph canvas and renders one row per entity type with no bound:

<div className="graph-legend">
  <div className="legend-title">Entity Types</div>
  {currentOntology.entityTypes.map(entity => ( ... ))}
</div>

There is no close, collapse, or move affordance anywhere — the only place it gets hidden is the mobile breakpoint, which drops it with display: none. On a large ontology it grows until it blankets the nodes it exists to annotate, which is what the reporter's screenshot shows.

The change

• Collapsible. A header button collapses the legend to a single pill showing the title and the entity-type count. The choice persists to localStorage, so it survives a reload.
• Sensible default for big ontologies. With no stored preference, the legend opens collapsed above LEGEND_AUTO_COLLAPSE_THRESHOLD (12) entity types, and re-applies that heuristic when a much larger ontology is loaded into an already-open session — so importing a big taxonomy doesn't bury the graph. An explicit toggle always wins over the heuristic.
• Bounded. The expanded body is capped at 45% of canvas height with internal scrolling, so even an unbounded entity list can't cover the graph.

Extracted into GraphLegend.tsx — it now owns state, and OntologyGraph is already ~590 lines and mounts cytoscape on render, which makes it impractical to reach in jsdom. OntologyGraph's diff is a one-line swap.

Accessibility: the toggle carries aria-expanded / aria-controls, plus an aria-label that keeps the visible "Entity Types" text inside the accessible name (WCAG 2.5.3) — without it the button announced as "Entity Types3". Only existing theme tokens are used, so the contrast suite is unaffected.

Verification

10 new unit tests in GraphLegend.test.tsx cover expand/collapse, persistence, the size heuristic, explicit-choice precedence, mid-session ontology growth, and localStorage being unavailable.

Also driven against the running app with Playwright:

Scenario	Result
Default ontology (6 types)	opens expanded, all 6 rows
Click toggle	collapses to the pill, 0 rows, persisted
Reload	still collapsed
community/1010yab/university-class-search (19 types)	opens collapsed
Expanding it	376px tall inside an 836px canvas, all 19 rows scrollable

No console errors in any state.

Notes

• Scoped to item 1 of Usability issues #87. Item 3 (empty PNG export) is already covered by fix: prevent empty PNG export for large graphs #95; item 2 (navigating large graphs) is a bigger design question I left alone.
• Independent of fix(lint): repair no-case-declarations errors and gate lint in CI #108 — no overlapping files.
• 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.