Give steering interactions a teal identity

[VolkerChristian]

Summary

• add a canonical teal interaction and identity palette
• distinguish nested steering prompts with You · steering
• apply teal consistently to pending animation, acknowledged surface, border, and title
• style the native Steer action with teal primary, hover, and pressed states
• expose and style the WebUI Steer action beside Stop during active turns
• retain blue for ordinary Send actions and owning turn-You cards
• document the steering presentation contract

Verification

• native test suite: 9/9 passed
• WebUI test suite: 77/77 passed
• Xvfb visual verification of nested teal steering card and active Steer button
• live pending-to-authoritative steering transition tested
• git diff --check