feat(sync): interactive setup, auth continuation, and credential resolution fixes (#70)

* feat(sync): add `setup` command to configure credentials interactively

Problem: users had to manually create a JSON credentials file at the
correct path before authenticating, with no guidance from the plugin.

Solution: add `OAuthClient:setup()` that prompts for client ID and
secret via `vim.ui.input`, writes to the shared
`google_credentials.json`, then immediately starts the OAuth flow.
Expose as `:Pending {gtasks,gcal} setup`. Also extend
`resolve_credentials()` to fall back to a shared `google_credentials.json`
so one file covers both backends.

* fix(sync): improve `setup` input loop with validation and masking

Problem: `setup()` used async `vim.ui.input` for both prompts, causing
newline and re-prompt issues when validation failed. The secret was also
echoed in plain text.

Solution: switch to synchronous `vim.fn.input` / `vim.fn.inputsecret`
loops with `vim.cmd.redraw()` + `nvim_echo` for inline error display and
re-prompting. Validate client ID format and `GOCSPX-` secret prefix
before saving.

* fix(oauth): fix `ipairs` nil truncation in `resolve_credentials` and add file-path setup option

Problem: `resolve_credentials` built `cred_paths` with a potentially nil
first element (`credentials_path`), causing `ipairs` to stop immediately
and always fall through to bundled placeholder credentials.

Solution: build `cred_paths` without nil entries using `table.insert`.
Also add a `2. Load from JSON file path` option to `setup()` via
`vim.fn.inputlist`, with `vim.fn.expand` for `~`/`$HOME` support and
the `installed` wrapper unwrap.

* doc: cleanup

* ci: format

lua/pending/sync/gtasks.lua

@@ -371,6 +371,10 @@ local function with_token(callback)
   end)
 end
 
+function M.setup()
+  client:setup()
+end
+
 function M.auth()
   client:auth()
 end