fix(planner): elevation chart drag survives leaving the canvas; move reset-zoom

- Drag-to-zoom on the chart now tracks via window mousemove/mouseup, so
  it keeps going when the pointer leaves the chart and still completes if
  released outside it (previously mouseleave aborted the drag and an
  outside release was lost). Listeners are detached on mouseup/unmount.
- Move the "reset zoom" button from top-right (which overlapped the
  color-mode dropdown) to the bottom-right of the chart, and restyle it
  on tokens.

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

apps/planner/app/components/SessionView.tsx

@@ -284,7 +284,7 @@ export function SessionView({ sessionId, hasJournalCallback, returnUrl, initialW
               {isZoomedByChart && (
                 <button
                   onClick={handleResetZoom}
-                  className="absolute right-3 top-1 z-10 rounded bg-white px-2 py-0.5 text-[11px] font-medium text-gray-600 shadow ring-1 ring-gray-200 hover:bg-gray-50"
+                  className="absolute bottom-7 right-3 z-10 rounded-md bg-bg-raised px-2 py-0.5 text-[11px] font-medium text-text-md shadow-sm ring-1 ring-border transition-colors hover:bg-bg-subtle"
                 >
                   {t("elevation.resetZoom", "Reset zoom")}
                 </button>