Add zoom (+/−), fit, and fullscreen controls to visual notes graph

[Copilot]

The graph view was fixed-height with no way to enlarge it, making dense graphs hard to read. This adds a compact toolbar with zoom and fullscreen controls directly in the graph header.

Controls added

• − / + — zoom out/in, scaling cy.zoom() by ÷1.25 / ×1.25 centred on the viewport midpoint
• ⊡ — fit to view, calls the existing refitGraph() to reset zoom and pan
• ⛶ / ✕ — fullscreen toggle; applies position: fixed; inset: 0 over the entire viewport, with the graph expanding to fill remaining height via flexbox; Escape exits (with preventDefault to stop propagation)

Implementation notes

• isFullscreen state persists across re-renders so the overlay survives sidecar-change reloads
• Fullscreen class and state are cleared in onunload to prevent a stuck overlay on component destruction
• Escape listener registered via Obsidian's registerDomEvent — auto-cleaned on unload, no manual teardown needed
• All buttons carry aria-label + title attributes

CSS

.visual-notes-fullscreen {
  position: fixed;
  inset: 0;
  z-index: var(--layer-notice, 200);
  display: flex;
  flex-direction: column;
}
.visual-notes-fullscreen .visual-notes-graph {
  flex: 1;
  height: auto;
  min-height: 0;
}

Tests

Six new source-contract assertions in renderer.test.ts cover: toolbar presence, zoom scaling factors, fullscreen class toggle, Escape handler, onunload cleanup, and button accessibility labels.