fix: handle missing :pending_batch_tasks assign in historical loader

Use Map.get with default for all loading_generation and pending_batch_tasks
socket.assigns accesses to prevent KeyError when key is not present.

lib/aprsme_web/live/map_live/historical_loader.ex

@@ -113,7 +113,7 @@ defmodule AprsmeWeb.MapLive.HistoricalLoader do
   @spec cancel_pending_loads(Socket.t()) :: Socket.t()
   def cancel_pending_loads(socket) do
     # Cancel any pending batch load messages
-    Enum.each(socket.assigns.pending_batch_tasks, &Process.cancel_timer/1)
+    Enum.each(Map.get(socket.assigns, :pending_batch_tasks, []), &Process.cancel_timer/1)
     assign(socket, :pending_batch_tasks, [])
   end
 
@@ -333,7 +333,7 @@ defmodule AprsmeWeb.MapLive.HistoricalLoader do
     generation = Map.get(socket.assigns, :loading_generation, 0)
     timer_ref = Process.send_after(self(), {:load_historical_batch, next_batch, generation}, 50)
 
-    assign(socket, :pending_batch_tasks, [timer_ref | socket.assigns.pending_batch_tasks])
+    assign(socket, :pending_batch_tasks, [timer_ref | Map.get(socket.assigns, :pending_batch_tasks, [])])
   end
 
   # Handle low zoom (heat map)