style: Refine form control visuals and interactions (#2606)

## Summary

- reduce horizontal padding for small, medium, and large buttons
- align medium and large input padding with the updated control spacing
- remove default shadows from buttons, toggles, inputs, selects,
comboboxes, pickers, checkboxes, radios, switches, and sliders
- retain shadows for elevated content such as dropdown menus, calendar
panels, and popovers

<img width="1260" height="967" alt="image"
src="https://github.com/user-attachments/assets/e87318ec-58bf-47e7-b79b-130b09fa927d"
/>
<img width="1260" height="967" alt="image"
src="https://github.com/user-attachments/assets/e0fcb3ef-c9ec-43db-81df-3d656bb5728b"
/>
<img width="1227" height="1100" alt="image"
src="https://github.com/user-attachments/assets/0f6795e6-6719-4975-8d24-1a3fdfbd377e"
/>

## Test Plan

- `cargo check -p gpui-component`
- `git diff --check`

Repository-wide `cargo fmt --all -- --check` is currently blocked by
pre-existing formatting drift in unrelated files.

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