Make the router effect run once and log uncaught exceptions with exc_info

[maartenbreddels]

Two changes from a production incident (reacton 1.10.3, widgetti/reacton#54).

Router effect

RoutingProvider re-queued solara.get_widget(main) in an effect without dependencies, on every render of the router. When a render pass aborted halfway, reacton before 1.10.3 could do that, the chained closure held an unreconciled main, get_widget raised from inside the effect, and the whole app was replaced by the traceback. reacton 1.10.3 fixes the abort; this makes the router not depend on it.

• solara.use_effect(get_nav_widget, []): the root element of the component is always the same VBox at the same slot, reacton updates it in place, so the Navigator widget is the same object for the lifetime of the app. The nav_widget.current.location = path line still runs on every render.
• get_widget(nav) instead of get_widget(main).children[0]. The old comment said it did not work; it does on reacton 1.10.3: nav is a widget element reconciled in the VBox's child context, which the downward search from the router context reaches. The stale comment is gone.
• Test: test_routing_provider_navigator_widget_stable pushes three paths and one browser-side location change, asserts the widget identity is stable and location follows every push. Rendered with handle_error=False, so a failing effect would raise.

Uncaught exception logging

FakeIPython.showtraceback logged "Uncaught exception: %s" with the formatted traceback in the message and no exception info. Error trackers group on the message, so every escaped event-handler exception of every app lands in one issue. Now logger.error("Uncaught exception", exc_info=value); the pdb branch and the control-socket message with the formatted traceback are unchanged. Test asserts the record carries the exception.

🤖 Generated with Claude Code

[maartenbreddels]

Why get_widget(nav) works now, and why it did not when the comment was written in 2023 (#81): reacton's get_widget only searched the current component context then, and nav is reconciled inside the VBox component's child context, hence the detour via get_widget(main).children[0]. reacton dfdca66, "get_widget did not always work for function components", released in v1.7.2, made it search child contexts. solara requires reacton>=1.9, so the detour is no longer needed on any supported reacton. The code comment now records this.