fix(kanban-dashboard): restore implementations dropped during salvages (#28481)

Four kanban dashboard test failures, all from PR salvages that picked up
the test additions but dropped the corresponding implementations.

- BOARD_COLUMNS: add 'review' (status added by PR f55d94a1e but the
  board API never grew the column → test_board_empty failed because
  VALID_STATUSES - {archived} mismatched the rendered columns).
- update_task: enrich the 'ready' 409 detail with the blocking parent
  list (id, title, status) and add _parents_blocking_ready helper.
  Implementation lost in the #26744 salvage (commit e215558ba) which
  pinned the test but not the server-side code.
- dist/index.js: add parseApiErrorMessage helper, wire it through the
  drag/drop banner, add patchErr state to the TaskDrawer and surface
  it inline by the action row. Lost in the same #26744 salvage.
- test_diagnostics_endpoint_severity_filter: update to at-or-above
  semantics (PR a94ddd807 changed the filter from exact-match so the
  warning filter now correctly includes error+critical too).

plugins/kanban/dashboard/plugin_api.py

@@ -137,7 +137,7 @@ def _conn(board: Optional[str] = None):
 # tasks into ``todo`` and makes the dashboard look like the Scheduled column
 # disappeared.
 BOARD_COLUMNS: list[str] = [
-    "triage", "todo", "scheduled", "ready", "running", "blocked", "done",
+    "triage", "todo", "scheduled", "ready", "running", "blocked", "review", "done",
 ]
 
 
@@ -683,6 +683,23 @@ def update_task(task_id: str, payload: UpdateTaskBody, board: Optional[str] = Qu
             else:
                 raise HTTPException(status_code=400, detail=f"unknown status: {s}")
             if not ok:
+                # For ``ready``, name the blocking parent(s) so the dashboard
+                # can render an actionable toast instead of a silent no-op.
+                # See #26744.
+                if s == "ready":
+                    blockers = _parents_blocking_ready(conn, task_id)
+                    if blockers:
+                        names = ", ".join(
+                            f"{p['title']!r} ({p['id']}, status={p['status']})"
+                            for p in blockers
+                        )
+                        raise HTTPException(
+                            status_code=409,
+                            detail=(
+                                f"Cannot move to 'ready': blocked by parent(s) "
+                                f"not done — {names}"
+                            ),
+                        )
                 raise HTTPException(
                     status_code=409,
                     detail=f"status transition to {s!r} not valid from current state",
@@ -747,6 +764,29 @@ def delete_task(task_id: str, board: Optional[str] = Query(None)):
         conn.close()
 
 
+def _parents_blocking_ready(
+    conn: sqlite3.Connection, task_id: str,
+) -> list:
+    """Return parent rows (``id``, ``title``, ``status``) that aren't ``done``
+    and therefore prevent ``task_id`` from being promoted to ``ready``.
+
+    Used to enrich the 409 response from :func:`update_task` so the
+    dashboard can show an actionable toast (#26744) instead of a silent
+    no-op.  Returns ``[]`` when nothing blocks the transition (e.g. no
+    parents, or all parents already done).
+    """
+    rows = conn.execute(
+        "SELECT t.id, t.title, t.status FROM tasks t "
+        "JOIN task_links l ON l.parent_id = t.id "
+        "WHERE l.child_id = ? AND t.status != 'done'",
+        (task_id,),
+    ).fetchall()
+    return [
+        {"id": r["id"], "title": r["title"], "status": r["status"]}
+        for r in rows
+    ]
+
+
 def _set_status_direct(
     conn: sqlite3.Connection, task_id: str, new_status: str,
 ) -> bool: