fix: restore immersive visual coverage

[rogerchappel]

Summary

• keep immersive agent mode fullscreen while preserving the single mounted VoiceAgent session
• avoid the composer backdrop-filter containing block around the fixed immersive overlay
• make the immersive visual backdrop opaque so chat text does not show through
• give Neural Constellation more depth by rotating the graph plane and distributing nodes across independent axes

Verification

• pnpm typecheck
• pnpm test src/lib/agent-visual-settings.test.ts
• pnpm lint:check (passes with existing warnings)
• git diff --check
• push hook: pnpm typecheck && pnpm build

Notes

• Fresh PR after fix: energize neural visual transitions #596 merged.
• Needs live QA for compact ↔ immersive fullscreen coverage and Neural Constellation spatial depth on the deployed preview/device.