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.

client/src/lib/api.ts

@@ -2514,6 +2514,8 @@ export const api = {
         | "stage"
         | "unstage"
         | "discard"
+        | "stage-hunk"
+        | "discard-hunk"
         | "branch-create"
         | "branch-delete",
       body: {
@@ -2523,6 +2525,9 @@ export const api = {
         base?: string;
         create?: boolean;
         paths?: string[];
+        path?: string;
+        hunk?: string;
+        context?: number;
         confirm?: boolean;
       } = {}
     ) =>