[codex] Add first-launch Stay intro

[avatarneil]

Summary

• add a first-launch full-screen intro that quiets scattered chatter into the Stay icon
• synthesize a short royalty-free Web Audio soundscape instead of shipping licensed audio
• expand the Tauri window during the intro, then restore the normal compact window
• persist completion so the intro only runs after the user first finishes or skips it

Validation

• bun run typecheck
• bun run test
• bun run frontend:build
• cargo check
• cargo test
• visual verification in the Codex in-app browser at http://127.0.0.1:1421/?launchIntro=1