fix(planner): keep journal callback token off the client (#2 Phase A)

The Save-to-Journal flow had the browser fetch the journal with a
\`Bearer \${callbackToken}\` header. The JWT was visible in DevTools,
exfiltratable via any XSS or browser extension, and the planner's
\`loader\` shipped it down to the client as part of the page payload.

Now:

- **New action**: \`POST /api/save-to-journal\` (\`routes/api.save-to-journal.ts\`).
  Body: \`{ sessionId, gpx }\`. The action loads \`callbackUrl\` +
  \`callbackToken\` from \`planner.sessions\` (set at /new time when the
  user came from the journal), POSTs to the journal server-to-server
  with the Bearer, and forwards the response.

- **\`SaveToJournalButton\`**: drops the \`callbackUrl\` + \`callbackToken\`
  props. Takes \`sessionId\` only and POSTs to the planner action.

- **\`session.\$id.tsx\` loader**: stops returning \`callbackUrl\` /
  \`callbackToken\` to the client. Returns a single \`hasJournalCallback\`
  boolean so the button still knows whether to render.

- **\`SessionView\`**: same prop simplification.

Trust model is unchanged: the same \`sessionId\` that grants Yjs
membership grants save authority. Knowing the URL = ability to act.
The action only adds a server-side hop so the JWT never reaches
browser JS.

Phase B (jti single-use enforcement on the journal side) follows in
a separate PR — needs a journal DB column + verifier change.

Full repo: pnpm typecheck / lint / test all green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/planner/app/routes.ts

@@ -7,5 +7,6 @@ export default [
   route("api/route", "routes/api.route.ts"),
   route("api/route-segments", "routes/api.route-segments.ts"),
   route("api/overpass", "routes/api.overpass.ts"),
+  route("api/save-to-journal", "routes/api.save-to-journal.ts"),
   route("session/:id", "routes/session.$id.tsx"),
 ] satisfies RouteConfig;