fix(git): the branch switcher lists and checks out branches only the remote has

GET /api/lanes/:id/git/branches read only refs/heads, so a branch someone
else pushed never showed after a fetch or pull, and `switch` refused it
(ENOBRANCH). It now also returns `remotes`: fetched remote-tracking branches
with no local branch of that name (origin/HEAD left out), listed under
"On the remote"; switching to one runs `git switch -c <b> --track
refs/remotes/<remote>/<b>` with the remote ref looked up, never passed
through (bare name only when exactly one remote has it, else EAMBIGUOUS).
The bar also re-reads the branch list after a fetch or pull instead of
keeping the list it loaded first.

server/openapi-extra/lanes.js

@@ -854,13 +854,13 @@ const paths = {
   "/api/lanes/{id}/git/branches": {
     get: {
       tags: ["Lanes"],
-      summary: "The lane repo's own local branches",
+      summary: "The lane repo's own local branches, and the remote ones not checked out yet",
       description:
-        "Feeds the lane's branch switcher. The repository comes from the lane row, never from the query — that is what GET /api/lanes/branches is for. Read-only, no same-origin guard.",
+        "Feeds the lane's branch switcher. `branches` are the local branches; `remotes` the fetched remote-tracking branches with no local branch of that name yet (`origin/feature`, the remotes' HEAD left out) — `switch` to one creates its local tracking branch. The repository comes from the lane row, never from the query — that is what GET /api/lanes/branches is for. Read-only, no same-origin guard.",
       operationId: "listLaneOwnBranches",
       parameters: [{ name: "id", in: "path", required: true, schema: { type: "integer" } }],
       responses: {
-        200: { description: "{available, branches, current, worktrees}." },
+        200: { description: "{available, branches, remotes, current, worktrees}." },
         404: { description: "Lane not found." },
       },
     },