fix: resolve remaining LuaLS type errors

Problem: CI lua-typecheck-action reported three categories of errors:
1. parse.lua - multi-assignment of tonumber() results left y/m/d typed
   as number? rather than integer, failing os.time()'s field types
2. gcal.lua - url_encode returned str:gsub() which yields string+integer
   but the annotation declared @return string (redundant-return-value)
3. gcal.lua - calendar_id typed string? from find_or_create_calendar was
   passed to functions expecting string; the existing `if err` guard did
   not narrow the type for LuaLS

Solution: replace the y/m/d multi-assignment with yn/mn/dn locals whose
types resolve cleanly to integer; wrap the gsub return in parentheses to
discard the count; add `or not calendar_id` to the error guard so LuaLS
narrows calendar_id to string for the rest of the scope.

lua/pending/sync/gcal.lua

@@ -99,9 +99,11 @@ end
 ---@param str string
 ---@return string
 local function url_encode(str)
-  return str:gsub('([^%w%-%.%_%~])', function(c)
-    return string.format('%%%02X', string.byte(c))
-  end)
+  return (
+    str:gsub('([^%w%-%.%_%~])', function(c)
+      return string.format('%%%02X', string.byte(c))
+    end)
+  )
 end
 
 ---@param method string
@@ -449,8 +451,8 @@ function M.sync()
   end
 
   local calendar_id, err = find_or_create_calendar(access_token)
-  if err then
-    vim.notify('pending.nvim: ' .. err, vim.log.levels.ERROR)
+  if err or not calendar_id then
+    vim.notify('pending.nvim: ' .. (err or 'calendar not found'), vim.log.levels.ERROR)
     return
   end