Markers: editable, manual x/y/z popup placement, drop heightmap auto-derive

Markers now carry a caller-supplied x/y/z instead of an auto-resolved
terrain height: clicking the map opens a real Leaflet popup pre-filled
with the clicked x/z, a default y of 60, and a random color, all
editable before saving. The same popup now also opens for editing an
existing marker (new updateMarker/PATCH /api/markers/:markerId, TDD'd
in markers.test.ts/index.test.ts). Removes the now-unused
resolveHeight/GET /api/height machinery. Linked-account markers
already synced server-side via the markers table, so cross-device
sync falls out of the existing loadMarkers()-on-init flow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tKdPZt78zbPUZMXWzKEKt

frontend/src/index.ts

@@ -14,6 +14,7 @@ const app = new Elysia()
   .get("/css/tailwind.css", () => Bun.file(join(import.meta.dir, "public/css/tailwind.css")))
   .get("/js/map.js", () => Bun.file(join(import.meta.dir, "public/js/map.js")))
   .get("/js/coords.js", () => Bun.file(join(import.meta.dir, "public/js/coords.js")))
+  .get("/js/colors.js", () => Bun.file(join(import.meta.dir, "public/js/colors.js")))
   .get("/js/mesh.js", () => Bun.file(join(import.meta.dir, "public/js/mesh.js")))
   .get("/js/mesh-format.js", () => Bun.file(join(import.meta.dir, "public/js/mesh-format.js")))
   .listen(Number(process.env.PORT ?? 3001));