refactor(sync): extract shared OAuth into oauth.lua

Problem: `gcal.lua` and `gtasks.lua` duplicated ~250 lines of identical
OAuth code (token management, PKCE flow, credential loading, curl
helpers, url encoding).

Solution: Extract a shared `OAuthClient` metatable in `oauth.lua` with
module-level utilities and instance methods. Both backends now delegate
all OAuth to `oauth.new()`. Skip `oauth` in `health.lua` backend
discovery by checking for a `name` field.

lua/pending/health.lua

@@ -65,7 +65,7 @@ function M.check()
     for _, path in ipairs(sync_paths) do
       local name = vim.fn.fnamemodify(path, ':t:r')
       local bok, backend = pcall(require, 'pending.sync.' .. name)
-      if bok and type(backend.health) == 'function' then
+      if bok and backend.name and type(backend.health) == 'function' then
         vim.health.start('pending.nvim: sync/' .. name)
         backend.health()
       end