feat(parse): flexible time parsing for @ suffix

Problem: the @HH:MM time suffix required zero-padded 24-hour format,
forcing users to write due:tomorrow@14:00 instead of due:tomorrow@2pm.

Solution: add normalize_time() that accepts bare hours (9, 14),
H:MM (9:30), am/pm (2pm, 9:30am, 12am), and existing HH:MM format,
normalizing all to canonical HH:MM on save.

lua/pending/init.lua

@@ -1,5 +1,4 @@
 local buffer = require('pending.buffer')
-local config = require('pending.config')
 local diff = require('pending.diff')
 local parse = require('pending.parse')
 local store = require('pending.store')
@@ -211,7 +210,7 @@ function M.prompt_date()
   if not id then
     return
   end
-  vim.ui.input({ prompt = 'Due date (YYYY-MM-DD[Thh:mm]): ' }, function(input)
+  vim.ui.input({ prompt = 'Due date (today, +3d, fri@2pm, etc.): ' }, function(input)
     if not input then
       return
     end