fix(desktop): order project-tree lanes by recency in the overview, not alphabetically

_build_repos emptied each lane's sessions array for the overview (hydrate=False)
payload BEFORE _sort_lanes ran. _lane_sort_key derives a lane's activity from
max(_session_time(s) for s in group["sessions"]), so with the rows already gone
every non-trunk lane scored activity=0.0 and the sort key
(is_trunk, is_kanban, -activity, label) collapsed to alphabetical-by-label. The
documented intent — branches and linked worktrees sort by most-recent activity,
then label — was silently defeated on the projects.tree RPC that feeds the
desktop sidebar overview, while the drill-in path (hydrate=True) kept the rows
and sorted correctly. Any repo with two or more non-trunk lanes showed a
different order in the overview than when opened.

Move the session-clearing to after _sort_lanes/_disambiguate_labels so the sort
reads real recency. Lane counts are still captured before clearing, so
sessionCount and the slim overview payload are unchanged — only the order is
fixed, and the overview now matches the drill-in.

tui_gateway/project_tree.py

@@ -376,8 +376,6 @@ def _build_repos(sessions: list[dict], resolve: Optional[Resolve], hydrate: bool
         group = entry["group"]
         group["sessions"].sort(key=_session_time, reverse=True)
         count = len(group["sessions"])
-        if not hydrate:
-            group["sessions"] = []
 
         repo_identity = _path_key(entry["repo_key"])
         repo = repos.get(repo_identity)
@@ -397,6 +395,15 @@ def _build_repos(sessions: list[dict], resolve: Optional[Resolve], hydrate: bool
     for repo in repo_list:
         repo["groups"] = _sort_lanes(repo["groups"])
         _disambiguate_labels(repo["groups"])
+        # Drop per-lane session rows only AFTER sorting: _lane_sort_key ranks
+        # non-trunk lanes by most-recent activity, which it derives from the
+        # session rows. Clearing them earlier makes every lane look inactive on
+        # the overview (hydrate=False) path and collapses the sort to
+        # alphabetical. Counts were already captured above, so the payload stays
+        # slim without losing the recency order.
+        if not hydrate:
+            for group in repo["groups"]:
+                group["sessions"] = []
     _disambiguate_labels(repo_list)
     return repo_list