fix(flowpilot): make action bar fixed to viewport bottom

Previous approach tried to fix the flex height chain, but it proved fragile
across grid → main → outlet → page transitions. New approach: action bar is
position:fixed at the bottom of the viewport, offset by sidebar width via
CSS variable --sidebar-w. Conversation area has pb-20 for clearance.

This works regardless of the height chain, email verification banner,
or any other variable-height elements above the session.

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

frontend/src/pages/FlowPilotSessionPage.tsx

@@ -139,11 +139,9 @@ export default function FlowPilotSessionPage() {
     )
   }
 
-  // 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.
+  // Active/completed session
   return (
-    <div className="flex h-[calc(100vh-56px)] flex-col overflow-hidden">
+    <div className="flex h-full flex-col">
       {/* Header */}
       <div
         className="flex items-center gap-3 border-b px-5 py-3 shrink-0"