fix(opencode): restore inner shadow in TUI exit monogram

[kitlangton]

TOP: Before 😭

BOTTOM: After 🥰

Summary

• restore the inner shadow for the single-symbol logo in the session exit banner
• add UI.logoMonogram() to render a typed 3-line monogram with the brighter palette used for this context
• remove fallback row logic in the exit banner and share glyph rendering logic via an internal draw(...) helper

Closes #12543

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