style: remove shadows from buttons and form controls

[ishigakijunichi]

Summary

Remove the shadow-xs from Button secondary and from the text-input style controls, so they sit flat.

• Button secondary: no shadow (default / destructive are unchanged).
• Shared controlBaseClasses (Input, Textarea, Field.Control, DateField input group): shadow removed in one place.
• Select trigger, Combobox, Autocomplete: shadow removed to match.

Button outline (its shadow and its fill) is handled in the companion PR #588. Not touched: Checkbox, Attachment, and the DataTable toolbar button groups still have their shadow.

Why

• Button secondary. It is a light-coloured button. A faint shadow on a pale fill blurs the outline of the UI and makes it look smudged rather than crisp.
• Input-style controls. Inputs, textareas, selects and the like already define their edge with a border-input border. Adding shadow-xs on top doubles the edge and softens it, which is the same blur problem as above. In dark mode the shadow is barely visible, so the two themes also looked different.
• Consistency. With flat controls, the surface (Card keeps its shadow-xs) is the raised layer and the controls inside it are flat. Buttons and inputs placed side by side share the same flat treatment, and all the input-like components (including Select, Combobox and Autocomplete) match each other.

Before / After

Screenshots are from the showcase app with the default theme.

button

Light

Dark

input

Light

Dark

textarea

Light

Dark

select

Light

Dark

combobox

Light

Dark

autocomplete

Light

Dark

Test plan

• pnpm vitest run in packages/core — snapshots updated; the only failures are the 17 pre-existing data-table localStorage tests, which fail identically on main
• Checked the examples/vite-app showcase pages (/showcase/primitives, /showcase/dropdown) in light and dark
• Changeset added (patch)