feat(lanes): stage/undo one hunk at a time, sticky review file headers

Review and the single-file view gain per-hunk Stage and Undo
(stage-hunk / discard-hunk: the hunk is named by its @@ line and
re-matched against a fresh diff, then git apply --cached / -R; undo
needs confirm: true and two clicks). Each review file header stays
pinned while its diff scrolls and carries the file's own
stage/unstage/discard.

server/routes/lanes.js

@@ -951,13 +951,19 @@ router.post("/:id/git/:action", sameOriginGuard, async (req, res) => {
       "EPROTECTED",
       "ESTAGEDOUTSIDE",
       "ENOREMOTE",
+      "EUNTRACKED",
     ];
     // A numeric code is git's own exit status (execFile reports one that way):
     // git ran and refused — a rejected push, a failed credential, a diverged
     // pull. That is the repository's state answering a valid request, not a
     // server fault, so it is a 409 under the string code the client expects.
     const exited = typeof err.code === "number";
-    const status = badRequest.includes(err.code) ? 400 : exited ? 409 : 500;
+    // EHUNKGONE is the same kind of answer: the tree moved under the click.
+    const status = badRequest.includes(err.code)
+      ? 400
+      : exited || err.code === "EHUNKGONE"
+        ? 409
+        : 500;
     // git's own stderr is the useful half of a failed push or pull (a
     // rejection, a missing credential, a diverged branch), so it joins the
     // message rather than riding a side field the client would have to know