Charts: direction arrows on shark move lines #182

What. Show which way a shark went on the move lines: an arrowhead (or chevrons along the line) pointing from the earlier capture's nursery to the later one.

Where.

• Each shark's page (/sharks/[id]), on the interactive map with Play and on the static MooreaMap. During Play, the line just drawn should point at the new capture site.
• /recaptures pools both directions on one line per nursery pair. Either draw two offset arrows sized by each direction's count, or keep one undirected line and leave direction to the popup (A → B: n · B → A: m). Pick whichever reads better.

Why. At the moment a line between two nurseries doesn't say which one the shark left (feedback on #178).

Acceptance

• Arrowheads use the line's own colour (species slot) and stay visible in light and dark themes.
• Both maps: Leaflet (an SVG marker or a small rotated marker at the midpoint or end, without a new third-party plugin unless it's added to the CSP and AGENTS.md) and the SSR MooreaMap (<marker> in the SVG).
• A→B then B→A on the same pair (for example 955000004999522: Apaura → Vaiane → Apaura) shows both directions, not one arrow on top of the other.
• Works with Charts: shortest water routes between nurseries (precomputed GeoJSON) for move lines and animation #183 ocean routes once they land (arrow follows the route's last segment).

Builds on #173 (PRs #178 and #179). Epic: #70