popup: Fix popup placement and base showcase startup (#2732)

## Summary
- position DatePicker, Select, and Combobox from their actual trigger
bounds
- make dropdown popups use side placement so Menu opens above/below the
trigger correctly
- keep the DropdownButton story's first example opening downward

## Root cause
Popup bounds were captured from a container that could include the
deferred popup, producing large offsets. Menu popups also used corner
positioning, which clamped an overflowing upward menu instead of using
side placement and flipping to the available side.

## Validation
- cargo fmt --all
- cargo test -p gpui-base positioner --lib
- cargo check -p gpui-component -p gpui-component-story
- git diff --check

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Co-authored-by: Codex <codex@openai.com>