refactor: charcoal palette + sidebar drawer fixes

- Switch from true-dark to charcoal palette (sidebar darkest, content lighter)
  Page #1a1c23, Sidebar #10121a, Card #22252e, Border #2e3240
- Update all Tailwind semantic mappings to match new palette
- Update landing page CSS hardcoded hex to new palette values
- Fix remaining hardcoded hex in SurveyResponsesPage, SessionsPanel, FlowPilotMessageBar
- Sidebar drawer starts below topbar (top: var(--topbar-h)) instead of viewport top
- Drawer section title uses amber (#fbbf24) for visual pop
- Unify all rail icons: white when inactive, cyan with bg-accent-dim when active

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

frontend/src/components/flowpilot/FlowPilotMessageBar.tsx

@@ -51,7 +51,7 @@ export function FlowPilotMessageBar({ onRespond, disabled = false, isProcessing
             ? 'border-border/50 opacity-50'
             : 'border-border focus-within:border-[rgba(6,182,212,0.3)]'
         )}
-        style={{ background: '#14161d' }}
+        style={{ background: 'var(--color-bg-card)' }}
       >
         <textarea
           ref={textareaRef}