Make submit shortcut configurable (or change default from `Ctrl+S`)

### Summary

In the stack submission UI, `Ctrl+S` is used to submit to GitHub. Could we make this shortcut configurable (and/or consider a different default)?

### Problem

`Ctrl+S` commonly maps to "save" in many terminal editors/tools, and in some terminal environments it can conflict with user expectations or existing keybindings. This can lead to accidental submissions or friction when switching contexts. In my case I was unable to submit my stack since `Ctrl+S` is being hijacked by my terminal and the `gh` TUI was never able to receive it.

### Proposal

One of the following (in order of preference):

1. Add configurable keybindings for TUI actions (including submit), with a sensible default.
2. Keep `Ctrl+S` as an optional binding but add an additional default (e.g. `Enter` on focused submit, `Ctrl+Enter`, or `s` in a specific mode).
3. If full keybinding customization is out of scope, switch the default submit shortcut to a less collision-prone key combo.

### Additional context

Current behavior observed in stack submission flow where `Ctrl+S` triggers submit.

Start by locating the stack submission UI and the handler for the current Ctrl+S shortcut, then inspect how gh-stack receives terminal input. Decide on the supported binding behavior from the proposal and verify that submission works with the chosen shortcut without relying on Ctrl+S.

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