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

README.md

@@ -1,6 +1,8 @@
 # pending.nvim
 
-Edit tasks like text. `:w` saves them.
+Edit tasks like text. Inspired by
+[oil.nvim](https://github.com/stevearc/oil.nvim),
+[vim-fugitive](https://github.com/tpope/vim-fugitive)
 
 ![demo](assets/demo.gif)
 
@@ -24,21 +26,6 @@ luarocks install pending.nvim
 :help pending.nvim
 ```
 
-## Icons
-
-All display characters are configurable. Defaults produce markdown-style checkboxes (`[ ]`, `[x]`, `[!]`):
-
-```lua
-vim.g.pending = {
-  icons = {
-    pending = ' ', done = 'x', priority = '!',
-    due = '.', recur = '~', category = '#',
-  },
-}
-```
-
-See `:help pending.Icons` for nerd font examples.
-
 ## Acknowledgements
 
 - [dooing](https://github.com/atiladefreitas/dooing)