feat(lanes): Add Lane repo/worktree mode toggle, manual branch, folder browse

- Repo mode adopts a directory as-is via /lanes/ensure (no worktree, no
  branch fields) - the right choice for a main repo you want stage
  detection on. Worktree mode (default) keeps the existing provisioning
  flow but now requires a manually-typed branch name instead of deriving
  one from the title.
- POST /lanes/worktree accepts an optional `branch`, validated via
  `git check-ref-format --branch`; omitting it preserves the CLI's
  existing auto-derived-branch behavior.
- New GET /lanes/browse lists a directory's immediate subdirectories,
  backing a small folder-browse modal on both path fields - browsers
  cannot expose an absolute path from a native picker, so this is
  server-backed instead, consistent with the tool's local-first model.

client/src/lib/api.ts

@@ -1922,11 +1922,30 @@ export const api = {
      * @param body The source repo, a slug/title, and the branch to fork from.
      * @returns `{ lane }` — the lane row created immediately, before provisioning finishes.
      */
-    worktree: (body: { sourceRepo: string; slug?: string; title?: string; base?: string }) =>
+    worktree: (body: {
+      sourceRepo: string;
+      slug?: string;
+      title?: string;
+      base?: string;
+      branch?: string;
+    }) =>
       request<{ lane: Lane }>("/lanes/worktree", {
         method: "POST",
         body: JSON.stringify(body),
       }),
+    /**
+     * GET /api/lanes/browse — list a directory's immediate subdirectories, for
+     * the Add Lane modal's folder browser. Browsers cannot expose absolute
+     * filesystem paths from a native picker, so this drives a server-backed one.
+     * @param path Absolute path to list; defaults server-side to the home dir.
+     * @returns `{ path, parent, entries }` — `parent` is null at the root.
+     */
+    browse: (path?: string) =>
+      request<{
+        path: string;
+        parent: string | null;
+        entries: { name: string; path: string; isGitRepo: boolean }[];
+      }>(`/lanes/browse${path ? `?path=${encodeURIComponent(path)}` : ""}`),
     /**
      * GET /api/lanes/:id — fetch one lane.
      * @param id The lane id.