fix: layer style modal above immersive mode

[rogerchappel]

Summary

• raise the Style modal overlay above immersive agent mode
• keep the fix scoped to the existing modal layer instead of changing voice session mounting

Verification

• pnpm typecheck
• git diff --check
• pre-commit hook: npx tsc --noEmit
• push hook: pnpm typecheck && pnpm build

Notes

• Fresh PR from latest main after the previous immersive visual branch.
• Needs quick visual QA: enter immersive mode, open Style, confirm the modal appears above the visual and remains usable on desktop/mobile.