Add mobile touch support for elevation chart interaction

- Single touch + drag: scrub the highlight point along the chart,
  map dot follows in real-time
- Tap: pan map to that point (10px threshold for movement)
- Two-finger touch: range select — area between fingers highlighted,
  on release zooms map to fit that route section
- touch-none CSS + preventDefault prevents page scrolling on chart

Spec updated with mobile touch interaction scenarios.

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

openspec/changes/elevation-map-interaction/specs/elevation-map-interaction/spec.md

@@ -29,3 +29,24 @@ Dragging a range on the elevation chart SHALL zoom the map to fit that section o
 #### Scenario: Reset zoom
 - **WHEN** the map has been zoomed via chart drag-select
 - **THEN** a reset button appears to return to the full route view
+
+### Requirement: Mobile touch interaction
+The elevation chart SHALL support touch-based interaction on mobile devices.
+
+#### Scenario: Single touch scrub
+- **WHEN** a user touches and drags on the chart with one finger
+- **THEN** the crosshair follows the finger position along the chart
+- **AND** the map highlight dot follows in real-time
+
+#### Scenario: Tap to pan
+- **WHEN** a user taps the chart (touch without significant movement)
+- **THEN** the map pans to that point along the route
+
+#### Scenario: Two-finger range select
+- **WHEN** a user places two fingers on the chart
+- **THEN** the area between the fingers is highlighted as a selection range
+- **AND** on release, the map zooms to fit the route coordinates in that range
+
+#### Scenario: No page scroll on touch
+- **WHEN** a user touches the elevation chart
+- **THEN** page scrolling is prevented (touch-none CSS + preventDefault)