fix(flowpilot): use calc(100vh-56px) for session page height — matches tree editor pattern

The flex height chain through grid → main → outlet → page doesn't reliably
constrain height, causing the action bar to overflow below viewport. The tree
editor already solves this with h-[calc(100vh-56px)] (subtracting the topbar).
Apply the same proven pattern to FlowPilotSessionPage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/FlowPilotSessionPage.tsx

@@ -139,9 +139,11 @@ export default function FlowPilotSessionPage() {
     )
   }
 
-  // Active/completed session
+  // Active/completed session — use calc(100vh-56px) to match tree editor pattern
+  // (56px = topbar height). This bypasses the flex height chain which doesn't
+  // reliably propagate through the grid → main → outlet → page chain.
   return (
-    <div className="flex h-full flex-col">
+    <div className="flex h-[calc(100vh-56px)] flex-col overflow-hidden">
       {/* Header */}
       <div
         className="flex items-center gap-3 border-b px-5 py-3 shrink-0"