Stabilize thread activity and prompt presentation

[VolkerChristian]

Summary

• keep root-thread admission and prompt-driven recency stable across native and WebUI
• fix exact prompt-editor expansion geometry and retain card lifecycle/status presentation
• shorten copied check confirmation from 1.5 seconds to 0.5 seconds in both frontends
• keep long executed-command text anchored at its beginning while retaining tail-follow exclusively for streaming output
• render transient native and WebUI notices as overlays without shifting message geometry

Verification

• native build passed
• native suite passed: 9/9
• WebUI suite passed: 82/82
• long executed-command regression test verifies the gray surface opens at its top
• notice geometry is identical before, during, and after dismissal
• production prompt expansion verified on Xvfb across forward and reverse boundary transitions