feat: time-aware due dates, persistent undo, @return audit

Problem: Due dates had no time component, the undo stack was lost on
restart and stored in a separate file, and many public functions lacked
required @return annotations.

Solution: Add YYYY-MM-DDThh:mm support across parse, views, recur,
complete, and init with time-aware overdue checks. Merge the undo stack
into the task store JSON so a single file holds all state. Add @return
nil annotations to all 27 void public functions across every module.

lua/pending/store.lua

@@ -19,6 +19,7 @@ local config = require('pending.config')
 ---@field version integer
 ---@field next_id integer
 ---@field tasks pending.Task[]
+---@field undo pending.Task[][]
 
 ---@class pending.store
 local M = {}
@@ -34,6 +35,7 @@ local function empty_data()
     version = SUPPORTED_VERSION,
     next_id = 1,
     tasks = {},
+    undo = {},
   }
 end
 
@@ -165,13 +167,24 @@ function M.load()
     version = decoded.version or SUPPORTED_VERSION,
     next_id = decoded.next_id or 1,
     tasks = {},
+    undo = {},
   }
   for _, t in ipairs(decoded.tasks or {}) do
     table.insert(_data.tasks, table_to_task(t))
   end
+  for _, snapshot in ipairs(decoded.undo or {}) do
+    if type(snapshot) == 'table' then
+      local tasks = {}
+      for _, raw in ipairs(snapshot) do
+        table.insert(tasks, table_to_task(raw))
+      end
+      table.insert(_data.undo, tasks)
+    end
+  end
   return _data
 end
 
+---@return nil
 function M.save()
   if not _data then
     return
@@ -182,10 +195,18 @@ function M.save()
     version = _data.version,
     next_id = _data.next_id,
     tasks = {},
+    undo = {},
   }
   for _, task in ipairs(_data.tasks) do
     table.insert(out.tasks, task_to_table(task))
   end
+  for _, snapshot in ipairs(_data.undo) do
+    local serialized = {}
+    for _, task in ipairs(snapshot) do
+      table.insert(serialized, task_to_table(task))
+    end
+    table.insert(out.undo, serialized)
+  end
   local encoded = vim.json.encode(out)
   local tmp = path .. '.tmp'
   local f = io.open(tmp, 'w')
@@ -300,6 +321,7 @@ function M.find_index(id)
 end
 
 ---@param tasks pending.Task[]
+---@return nil
 function M.replace_tasks(tasks)
   M.data().tasks = tasks
 end
@@ -325,11 +347,24 @@ function M.snapshot()
   return result
 end
 
+---@return pending.Task[][]
+function M.undo_stack()
+  return M.data().undo
+end
+
+---@param stack pending.Task[][]
+---@return nil
+function M.set_undo_stack(stack)
+  M.data().undo = stack
+end
+
 ---@param id integer
+---@return nil
 function M.set_next_id(id)
   M.data().next_id = id
 end
 
+---@return nil
 function M.unload()
   _data = nil
 end