Add multi-day data model, computeDays, and GPX roundtrip

- overnight.ts: Yjs helpers to set/clear/check overnight flag on waypoints
- compute-days.ts: Pure function splitting routes into DayStage[] at day breaks
- use-days.ts: Reactive hook mapping Yjs state into computeDays() input
- GPX generate: emit <type>overnight</type> for isDayBreak waypoints
- GPX parse: recognize <type>overnight</type> and set isDayBreak on waypoints

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

apps/planner/app/lib/overnight.ts

@@ -0,0 +1,24 @@
+import * as Y from "yjs";
+import type { YjsState } from "./use-yjs.ts";
+
+/**
+ * Set or clear the overnight flag on a waypoint.
+ */
+export function setOvernight(yjs: YjsState, index: number, value: boolean): void {
+  const waypointMap = yjs.waypoints.get(index);
+  if (!waypointMap) return;
+  yjs.doc.transact(() => {
+    if (value) {
+      waypointMap.set("overnight", true);
+    } else {
+      waypointMap.delete("overnight");
+    }
+  }, "local");
+}
+
+/**
+ * Check if a waypoint Y.Map has the overnight flag set.
+ */
+export function isOvernight(yMap: Y.Map<unknown>): boolean {
+  return yMap.get("overnight") === true;
+}

apps/planner/app/lib/use-days.ts

@@ -0,0 +1,68 @@
+import { useEffect, useState } from "react";
+import * as Y from "yjs";
+import { computeDays, type DayStage } from "@trails-cool/gpx";
+import type { Waypoint } from "@trails-cool/types";
+import type { TrackPoint } from "@trails-cool/gpx";
+import type { YjsState } from "./use-yjs.ts";
+import { isOvernight } from "./overnight.ts";
+
+/**
+ * Reactive hook that computes day stages from Yjs waypoints and route data.
+ * Returns an empty array for single-day routes (no overnight waypoints).
+ */
+export function useDays(yjs: YjsState | null): DayStage[] {
+  const [days, setDays] = useState<DayStage[]>([]);
+
+  useEffect(() => {
+    if (!yjs) return;
+
+    const recompute = () => {
+      // Extract waypoints with isDayBreak from Yjs
+      const waypoints: Waypoint[] = yjs.waypoints.toArray().map((yMap: Y.Map<unknown>) => ({
+        lat: yMap.get("lat") as number,
+        lon: yMap.get("lon") as number,
+        name: yMap.get("name") as string | undefined,
+        isDayBreak: isOvernight(yMap) || undefined,
+      }));
+
+      // Check if any waypoint has isDayBreak
+      const hasBreaks = waypoints.some((w) => w.isDayBreak);
+      if (!hasBreaks) {
+        setDays([]);
+        return;
+      }
+
+      // Extract track points from route coordinates stored in Yjs
+      const coordsStr = yjs.routeData.get("coordinates") as string | undefined;
+      if (!coordsStr) {
+        setDays([]);
+        return;
+      }
+
+      try {
+        // coordinates are stored as [[lon, lat, ele], ...] (GeoJSON format)
+        const coords: number[][] = JSON.parse(coordsStr);
+        const trackPoints: TrackPoint[] = coords.map((c) => ({
+          lat: c[1]!,
+          lon: c[0]!,
+          ele: c[2],
+        }));
+
+        setDays(computeDays(waypoints, [trackPoints]));
+      } catch {
+        setDays([]);
+      }
+    };
+
+    yjs.waypoints.observeDeep(recompute);
+    yjs.routeData.observe(recompute);
+    recompute();
+
+    return () => {
+      yjs.waypoints.unobserveDeep(recompute);
+      yjs.routeData.unobserve(recompute);
+    };
+  }, [yjs]);
+
+  return days;
+}