Open Settings from the shell's own buttons without crashing

[venator]

Clicking the Settings cog on the rail crashes the window when Settings is not already open: cannot read tinyclient::gui::state::Shell while it is already being updated, from SettingsWindow::page. The click runs inside the shell's update, and gpui draws a new window once before open_window returns. That draw reads the shell, for the accounts and whether chatd answers. "Settings…" in the accounts menu and "Call settings…" in a call's menu take the same path. The menu bar and ⌘, do not, which is why they work.

settings::open now opens the window once the current update is over (cx.defer), so every caller is covered. That also covers a window already open being moved to another section, which saved the choice through the shell.

There is no unit test: nothing in the crate opens a window under test. Instead, the --settings shot now opens Settings from inside the shell's update, as the buttons do. Without the fix it panics with the same trace; with it, it runs to the end. cargo test --workspace and cargo test -p tinyclient --features gui pass.

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