feat(parse): add parse_duration_to_days for duration string conversion

Problem: The archive command accepted only a bare integer for days,
inconsistent with the `+Nd`/`+Nw`/`+Nm` duration syntax used elsewhere.

Solution: Add `parse_duration_to_days()` supporting `Nd`, `Nw`, `Nm`,
and bare integers. Returns nil on invalid input for caller error handling.

lua/pending/parse.lua

@@ -667,4 +667,29 @@ function M.is_today(due)
   return time_part >= current_time
 end
 
+---@param s? string
+---@return integer?
+function M.parse_duration_to_days(s)
+  if s == nil or s == '' then
+    return nil
+  end
+  local n = s:match('^(%d+)d$')
+  if n then
+    return tonumber(n) --[[@as integer]]
+  end
+  n = s:match('^(%d+)w$')
+  if n then
+    return tonumber(n) --[[@as integer]] * 7
+  end
+  n = s:match('^(%d+)m$')
+  if n then
+    return tonumber(n) --[[@as integer]] * 30
+  end
+  n = s:match('^(%d+)$')
+  if n then
+    return tonumber(n) --[[@as integer]]
+  end
+  return nil
+end
+
 return M