feat(gtasks): add Google Tasks bidirectional sync

Problem: pending.nvim only supported one-way push to Google Calendar.
Users who use Google Tasks had no way to sync tasks bidirectionally.

Solution: add `lua/pending/sync/gtasks.lua` backend with OAuth PKCE
auth, push/pull/sync actions, and field mapping between pending tasks
and Google Tasks (category↔tasklist, `priority`/`recur` via notes).

lua/pending/config.lua

@@ -10,8 +10,12 @@
 ---@field calendar? string
 ---@field credentials_path? string
 
+---@class pending.GtasksConfig
+---@field credentials_path? string
+
 ---@class pending.SyncConfig
 ---@field gcal? pending.GcalConfig
+---@field gtasks? pending.GtasksConfig
 
 ---@class pending.Keymaps
 ---@field close? string|false