Fix cards example: fold select side-effects into reactiveProxy setters

[khusmann]

Problem

The Dynamic Column Cards example (examples/cards.R) errored at render:

change is claimed by both a value binding (value/checked) and an explicit on* handler on the same element.

Both <select> elements bound value = and attached an onChange handler. irid enforces one channel per DOM event (see enforce_one_channel_per_event in R/process_tags.R): a change event is either bound or handled, never both.

Fix

Fold each select's onChange side-effects into the set of a reactiveProxy, which is the framework's idiom for running a synchronous side-effect on write:

• Dataset select: the setter writes dataset_name and resets selected_columns/choice.
• Add-a-column select: the setter appends the chosen column to selected_columns and always snaps choice back to "" (the placeholder), replacing the old event$value read with the incoming value.

Verification

Launched the app and confirmed the page renders with no one-channel error.