feat(webui): add interactive particle hero background

[chengyongru]

Summary

• add a lazy-loaded Canvas UI Particle Object background to the empty-thread hero
• tune the particle scale, opacity, pointer response, and spring motion for the nanobot mark
• use a lighter mobile profile while preserving vertical touch scrolling
• keep the greeting and composer layered above the interactive canvas
• document the bundled Canvas UI code under its MIT + Commons Clause terms

Validation

• cd webui && bun run lint
• cd webui && bun run test (52 files, 782 tests)
• cd webui && bun run build

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License note

ParticleObject.tsx is adapted from DavidHDev/canvas-ui. It remains covered by the MIT + Commons Clause license recorded in THIRD_PARTY_NOTICES.md; the rest of nanobot remains under its existing license.