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/buffer.lua

@@ -37,10 +37,12 @@ function M.current_view_name()
   return current_view
 end
 
+---@return nil
 function M.clear_winid()
   task_winid = nil
 end
 
+---@return nil
 function M.close()
   if task_winid and vim.api.nvim_win_is_valid(task_winid) then
     vim.api.nvim_win_close(task_winid, false)
@@ -79,6 +81,7 @@ local function setup_syntax(bufnr)
 end
 
 ---@param above boolean
+---@return nil
 function M.open_line(above)
   local bufnr = task_bufnr
   if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
@@ -205,6 +208,7 @@ local function restore_folds(bufnr)
 end
 
 ---@param bufnr? integer
+---@return nil
 function M.render(bufnr)
   bufnr = bufnr or task_bufnr
   if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
@@ -249,6 +253,7 @@ function M.render(bufnr)
   restore_folds(bufnr)
 end
 
+---@return nil
 function M.toggle_view()
   if current_view == 'category' then
     current_view = 'priority'