Fix waypoint loading in Edit in Planner flow

Three issues fixed:

1. GPX parser used browser DOMParser which doesn't exist in Node/Vite SSR.
   Added async parseGpxAsync() using linkedom for server-side parsing.

2. Server-side session initialization stored waypoints in a Yjs doc
   instance separate from the Vite WebSocket plugin's doc store.
   Moved waypoint initialization to client-side: API returns parsed
   waypoints, client adds them to Yjs after sync.

3. GPX was encoded in URL params causing HTTP 431. Now the Journal
   creates a Planner session via API (POST body), and only passes
   compact waypoint coordinates in URL params.

Verified: Journal route → Edit in Planner → 4 waypoints loaded,
route computed (79.3km), elevation profile, Save to Journal ready.

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

packages/gpx/src/index.ts

@@ -1,3 +1,3 @@
-export { parseGpx } from "./parse";
+export { parseGpx, parseGpxAsync } from "./parse";
 export { generateGpx } from "./generate";
 export type { GpxData, TrackPoint, ElevationProfile } from "./types";

packages/gpx/src/parse.ts

@@ -4,9 +4,39 @@ import type { GpxData, TrackPoint, ElevationProfile } from "./types";
 /**
  * Parse a GPX XML string into structured data.
  */
+let _LinkedDOMParser: typeof DOMParser | null = null;
+
+async function getDOMParser(): Promise<typeof DOMParser> {
+  if (typeof DOMParser !== "undefined") return DOMParser;
+  if (!_LinkedDOMParser) {
+    const linkedom = await import("linkedom");
+    _LinkedDOMParser = linkedom.DOMParser as unknown as typeof DOMParser;
+  }
+  return _LinkedDOMParser;
+}
+
 export function parseGpx(xml: string): GpxData {
-  const parser = new DOMParser();
-  const doc = parser.parseFromString(xml, "application/xml");
+  // Synchronous path for browser
+  if (typeof DOMParser !== "undefined") {
+    return parseGpxWithParser(new DOMParser(), xml);
+  }
+  // Fallback: try linkedom synchronously (works in Node with top-level await or CJS)
+  try {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    const { DOMParser: LP } = require("linkedom");
+    return parseGpxWithParser(new LP() as unknown as DOMParser, xml);
+  } catch {
+    throw new Error("DOMParser not available — install linkedom for Node.js");
+  }
+}
+
+export async function parseGpxAsync(xml: string): Promise<GpxData> {
+  const Parser = await getDOMParser();
+  return parseGpxWithParser(new Parser(), xml);
+}
+
+function parseGpxWithParser(parser: DOMParser, xml: string): GpxData {
+  const doc = parser.parseFromString(xml, "application/xml") as unknown as Document;
 
   const parserError = doc.querySelector("parsererror");
   if (parserError) {