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/pages/admin/SurveyResponsesPage.tsx

@@ -100,7 +100,7 @@ function ExpandedDetail({ response }: { response: SurveyResponseDetail }) {
                 key={q.id}
                 className="rounded-lg p-4"
                 style={{
-                  background: '#14161d',
+                  background: 'var(--color-bg-card)',
                   border: '1px solid var(--glass-border)',
                 }}
               >
@@ -223,7 +223,7 @@ function ResponseRow({
               <div className="fixed inset-0 z-40" onClick={() => setShowMenu(false)} />
               <div
                 className="absolute right-3 top-full z-50 mt-1 w-44 rounded-xl py-1 shadow-xl"
-                style={{ background: '#14161d', border: '1px solid var(--glass-border)', }}
+                style={{ background: 'var(--color-bg-card)', border: '1px solid var(--glass-border)', }}
               >
                 <button
                   onClick={() => { onMarkRead(); setShowMenu(false) }}