Truncation Logic Refinement

Nucleic-Session: FE1E2C94-7945-4827-AF56-74525A9BD434
Co-authored-by: Nucleic <[email protected]>

NucleicRemote/NucleicRemote/Views/SessionDetailView.swift

@@ -420,6 +420,15 @@ struct SessionDetailView: View {
     }
 }
 
+/// The transcript's measured height, fed to the floating action card so it can bound itself to the
+/// screen (see `availableHeight`).
+private struct AvailableHeightKey: PreferenceKey {
+    static var defaultValue: CGFloat { 0 }
+    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
+        value = max(value, nextValue())
+    }
+}
+
 struct TranscriptList: View {
     let events: [AgentEvent]
     /// Bumped by the parent when the user sends a message — a deliberate "show me what happens