Phase 4: web markers and JourneyMap/Xaero waypoint chat sharing

Linked accounts can place 2D markers on the map (y auto-derived from the
chunk store's heightmap); anonymous visitors keep a localStorage-only
list via the same height lookup. Sharing a marker forwards a structured
payload to the mod over its WS connection, which builds the actual
chat text (see MCMapper-Mod for the JourneyMap/Xaero formatting).

Built test-first per the project's TDD workflow.

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

frontend/src/index.ts

@@ -13,6 +13,7 @@ const app = new Elysia()
   .get("/health", () => ({ status: "ok" }))
   .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/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));