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/dashboard/SessionsPanel.tsx

@@ -28,7 +28,7 @@ export function SessionsPanel({ sessions, delay = 200 }: SessionsPanelProps) {
           View All
         </Link>
       </div>
-      <div className="divide-y divide-[#1e2130]">
+      <div className="divide-y divide-border">
         {sessions.map(session => (
           <Link
             key={session.id}