feat(planner): collapsible elevation chart with summary bar

Docked-but-collapsible placement (the chosen direction):

- A collapse toggle (chevron) in the chart header; state persists to
  localStorage.
- Collapsed = a thin summary bar: a mini sage sparkline + distance and
  ascent/descent + an expand toggle. Reclaims map space on demand.
- Summary figures come from the same authoritative routeStats the
  sidebar uses (distance, elevationGain, elevationLoss), so the numbers
  match the sidebar exactly — not recomputed from the raw elevation
  points (which over-counted ascent ~2x from noise).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/planner/app/components/SessionView.tsx

@@ -280,7 +280,7 @@ export function SessionView({ sessionId, hasJournalCallback, returnUrl, initialW
           </div>
           <Suspense fallback={null}>
             <div className="relative">
-              <ElevationChart yjs={yjs} onHover={handleElevationHover} highlightDistance={highlightChartDistance} onClickPosition={handleChartClick} onDragSelect={handleChartDragSelect} days={days} />
+              <ElevationChart yjs={yjs} onHover={handleElevationHover} highlightDistance={highlightChartDistance} onClickPosition={handleChartClick} onDragSelect={handleChartDragSelect} days={days} routeStats={routeStats} />
               {isZoomedByChart && (
                 <button
                   onClick={handleResetZoom}