feat(sync): unify Google auth under :Pending auth

Problem: users had to run `:Pending gtasks auth` and `:Pending gcal
auth` separately, producing two token files and two browser consents
for the same Google account.

Solution: introduce `oauth.google_client` with combined tasks +
calendar scopes and a single `google_tokens.json`. Remove per-backend
`auth`/`setup` from `gcal` and `gtasks`; add top-level `:Pending auth`
that prompts with `vim.ui.select` and delegates to the shared client's
`setup()` or `auth()` based on credential availability.

lua/pending/sync/oauth.lua

@@ -501,5 +501,14 @@ end
 
 M._BUNDLED_CLIENT_ID = BUNDLED_CLIENT_ID
 M._BUNDLED_CLIENT_SECRET = BUNDLED_CLIENT_SECRET
+M.BUNDLED_CLIENT_ID = BUNDLED_CLIENT_ID
+
+M.google_client = M.new({
+  name = 'google',
+  scope = 'https://www.googleapis.com/auth/tasks'
+    .. ' https://www.googleapis.com/auth/calendar',
+  port = 18392,
+  config_key = 'google',
+})
 
 return M