chore(ui/dashboard): replace watch() with getValues/useWatch in form handlers

[steveninhle]

Summary

• Replace inline watch() calls inside event handlers with getValues() in VariationsSwitch — eliminates 6 reactive subscriptions that only needed a point-in-time read
• Combine duplicate watch('environmentId') calls into a single subscription in PushCreateUpdateModal
• Replace watch('featureId') with useWatch in ExperimentCreateUpdateModal and ExperimentSettings to scope re-renders to only those components
• Replace watch('variations') with useWatch in DefaultVariations for the same reason

Why

watch('field') called at the top level of a component (or inside a handler) registers a reactive subscription: every time that field changes, the entire component re-renders. In handlers, only the current value is needed — no subscription required. getValues() reads the value without subscribing. For render-time values that belong to a child/leaf component, useWatch confines the re-render to that component instead of bubbling it up to the parent.