feat(views): make queue view sort order configurable (#154)

Problem: the queue/priority view sort in `sort_tasks_priority()` uses a
hardcoded tiebreak chain (status, priority, due, order, id). Users who
care more about due dates than priority have no way to reorder it.

Solution: add `view.queue.sort` config field (string[]) that defines an
ordered tiebreak chain. `build_queue_comparator()` maps each key to a
comparison function and returns a single comparator. Unknown keys emit a
`log.warn`. The default matches the previous hardcoded behavior.

lua/pending/config.lua

@@ -87,6 +87,7 @@
 ---@field hide_done_categories? boolean
 
 ---@class pending.QueueViewConfig
+---@field sort? string[]
 
 ---@class pending.ViewConfig
 ---@field default? 'category'|'priority'
@@ -133,7 +134,9 @@ local defaults = {
       folding = true,
       hide_done_categories = false,
     },
-    queue = {},
+    queue = {
+      sort = { 'status', 'priority', 'due', 'order', 'id' },
+    },
   },
   keymaps = {
     close = 'q',