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/components/flowpilot/FlowPilotSession.tsx

@@ -193,8 +193,8 @@ export function FlowPilotSession({
 
       {/* Main content area: conversation + sidebar */}
       <div className="flex flex-1 min-h-0">
-        {/* Conversation column */}
-        <div ref={scrollRef} className="flex-1 overflow-y-auto p-3 sm:p-4 lg:p-6">
+        {/* Conversation column — pb-20 provides clearance for the fixed action bar */}
+        <div ref={scrollRef} className="flex-1 overflow-y-auto p-3 pb-20 sm:p-4 sm:pb-20 lg:p-6 lg:pb-20">
           <div className="mx-auto max-w-2xl space-y-3">
             {allSteps.map((step) => (
               <FlowPilotStepCard