feat(webui): add interactive particle hero background

[chengyongru]

Summary

• add a lazy-loaded Canvas UI particle background to the empty-thread hero
• tune particle scale, opacity, pointer response, spring motion, and compact mobile behavior
• keep the greeting and composer layered above the interactive canvas while preserving the current message-thread layout
• use the current bundled nanobot SVG and preserve vertical touch scrolling after OrbitControls initializes
• document the bundled Canvas UI code under its MIT + Commons Clause terms

Validation

• npm ci --ignore-scripts --dry-run
• bun install --frozen-lockfile
• bun run lint
• bun run test (64 files, 1,006 tests)
• bun run build
• isolated gateway + headless browser at 1280×720 and 390×844: particle state reached ready, desktop/compact profiles selected correctly, canvas retained touch-action: pan-y, foreground controls stayed visible, and no console errors were reported

Review notes

• Draft port of chengyongru/nanobot#315, rebased onto the current HKUDS/nanobot:main.
• The bundled component is covered by MIT + Commons Clause terms recorded in THIRD_PARTY_NOTICES.md. Please confirm that the additional restriction is acceptable before marking this ready or merging.
• The lazy ParticleObject production chunk is about 649.5 kB (168.0 kB gzip), above Vite's warning threshold. It is excluded from the initial bundle, but the empty-thread download/runtime cost should be reviewed.