feat(demo): improve cursor movement and typing realism

[gregpriday]

Summary

• Replaces fixed-duration cursor movement with Fitts's Law scaling so short moves to large targets are fast and long moves to small targets decelerate naturally
• Adds Perlin-noise micro-jitter (~2px amplitude) over interpolated x/y coordinates for organic hand tremor without altering the overall path
• Splits long movements (>500px) into a ballistic phase and an acquisition phase, with an explicit overshoot and correction curve on moves over 500px
• Adapts step count to distance and duration rather than always using 30 keyframes
• Replaces uniform 1000 / cps keystroke delay with Gaussian-distributed base delays, longer pauses after spaces and punctuation, burst-mode multiplier for mid-word characters, and occasional hesitation pauses
• Adds a brief cursor-settling drift after clicks to simulate the hand relaxing post-click

Resolves #4776

Changes

• src/components/Demo/DemoCursor.tsx — all movement, typing, and click-feedback logic
• src/components/Demo/__tests__/DemoCursor.test.tsx — test coverage for the new behaviours
• Minor type and preload adjustments in electron/preload.cts, shared/types/ipc/api.ts, and shared/types/ipc/demo.ts

Testing

Unit tests updated and passing. The Fitts's Law scaling, noise generation, two-phase movement, variable keystroke timing, and click-settling are all covered in the test suite.