feat: explain 3D graph touch rotation

[atomantic]

Summary

• Show a brief, non-blocking “Drag to rotate” cue on the first touch of each 3D Brain and Memory graph.
• Keep one-finger rotation intact and share the one-time timeout behavior between both canvases.
• Cover first-touch, timeout, and mouse-input behavior with a focused hook test.

Test plan

• cd client && npm test -- src/hooks/useFirstTouchHint.test.jsx src/hooks/index.test.js
• cd client && npx biome lint --error-on-warnings src/hooks/useFirstTouchHint.js src/hooks/useFirstTouchHint.test.jsx src/components/brain/tabs/BrainGraph.jsx src/components/cos/tabs/MemoryGraph.jsx
• cd client && npm run build

Closes #4169

[atomantic]

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