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.
Barrett Ruth
67aa8d71e6