## Why

Long routes -- multi-day bike tours, thru-hikes, extended backpacking trips --
are flat lists of waypoints with no structure. Users planning a 5-day ride from
Berlin to Prague have no way to mark where each day ends, see per-day distance
and climbing, or reason about daily effort. They resort to external spreadsheets
or mental arithmetic to divide the route into manageable stages.

The Planner already computes total distance and elevation across the full route.
Adding day structure is a matter of marking overnight stops on existing waypoints
and deriving per-day stats from the segment data we already have.

## What Changes

- **Day-break waypoints**: Any waypoint can be toggled as an overnight stop.
  This adds an `overnight: true` flag to the waypoint's Y.Map in the existing
  Yjs waypoints array. First and last waypoints are implicit day boundaries.
- **Per-day stats**: Distance, total ascent, and estimated duration computed per
  day by splitting the route at overnight waypoints. Derived from the existing
  `segmentBoundaries` and `coordinates` in the enriched route data.
- **Sidebar day breakdown**: Waypoints grouped by day with collapsible sections,
  per-day stats, and overnight toggle. Day 1 expanded by default.
- **Elevation chart day dividers**: Dashed vertical lines at day boundaries with
  "Day N" labels.
- **Map day labels**: White pill markers on the route at day boundaries showing
  "Day 1 . 120 km".
- **GPX export**: Day-break waypoints exported with a `<type>overnight</type>`
  element so the structure survives round-trips.

All state lives in Yjs. No database changes are needed -- the Planner remains
stateless. The visual design is already specified in the `visual-redesign`
change (D4 sidebar, D5 map markers, D6 elevation chart); this spec covers
the data model, computation logic, and integration wiring.

## Capabilities

### New Capabilities

- `multi-day-routes`: Overnight waypoint markers, per-day stats computation,
  day-aware sidebar/chart/map display, multi-day GPX export

### Modified Capabilities

- `planner-session`: Waypoints gain an `overnight` property in Yjs state
- `map-display`: Day boundary labels on route, overnight marker styling
- `gpx-export`: Day-break metadata in exported GPX waypoints

## Non-Goals

- **Automatic day splitting**: No algorithm to suggest where to stop. Users
  decide manually. This avoids opinionated defaults and keeps the logic simple.
- **Accommodation search**: No POI lookup for campsites or hotels. Out of scope.
- **Per-day routing profiles**: All days use the same BRouter profile. Supporting
  different profiles per day would require rearchitecting the routing pipeline.
- **Journal integration**: Saving multi-day routes to the Journal is a separate
  concern (route-features change). This spec is Planner-only.

## Impact

- **Yjs state**: `overnight` boolean added to waypoint Y.Map entries (additive,
  backwards-compatible -- existing sessions without it behave as single-day)
- **Shared types**: `Waypoint.isDayBreak` already exists in `@trails-cool/types`
  but is unused. This change activates it.
- **New utility**: `compute-days.ts` -- pure function that splits route data at
  overnight waypoints and returns per-day stats
- **Sidebar**: `WaypointSidebar.tsx` gains day-grouped view with collapsible
  sections and overnight toggle buttons
- **ElevationChart**: Canvas drawing extended with vertical day dividers
- **Map**: New day-label layer and overnight marker variant
- **GPX**: `generateGpx` extended to emit `<type>overnight</type>` on day-break
  waypoints
- **i18n**: New keys for day labels, overnight toggle, per-day stats (en + de)