Implement Gate mission selection and prototype mission card UI

[JOY]

Summary

Add the first UI for choosing a Gate mission from the Relay Yard loop.

Scope

• Add a mission board or mission card panel.
• Show mission title, risk, expected TIME cost, reward, recommended role, and hidden objective placeholder.
• Allow selecting current Frame and optional NPC ally placeholder.
• Keep mission start as a prototype transition or debug action until the Gate instance exists.

Acceptance Criteria

• Player can open a Gate mission selection surface in Play Mode.
• At least one mission card uses the Underpass Signal direction.
• Mission card clearly communicates risk and reward.
• UI does not mutate rewards or mission success client-side.
• Selected mission can be recorded as a pending server-owned mission choice or local prototype placeholder with clear debug labeling.

References

• docs/design/25-core-loop-v1.md
• docs/design/15-gate-dungeon-pioneer-charter-system.md

[JOY]

Implemented and merged in PR #165. Current root validation after PR #167 still compiles and enters Play Mode cleanly; Mission Board remains part of the active prototype HUD.