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>

package.json

@@ -44,6 +44,7 @@
     "i18next-browser-languagedetector": "^8.2.1",
     "jsdom": "^29.0.1",
     "leaflet": "^1.9.4",
+    "linkedom": "^0.18.12",
     "playwright": "^1.58.2",
     "postgres": "^3.4.8",
     "prettier": "^3.8.1",