fix: Form primitives spacing

[gcgoncalves]

Relates to IBM/mcp-context-forge#6508

Three root causes of inconsistent label-to-control gaps across forms:

• Label: leading-none lived in the CVA base string, so a call-site text-sm silently stripped it via tailwind-merge (line-height utilities are treated as conflicting with font-size ones). Moved to a [data-slot="label"] CSS rule that no className can strip.
• SelectTrigger: height was set via data-[size=*], an attribute selector that always beats a plain utility class in specificity, so call-site h-10 overrides were silently ignored. Replaced with a CVA size variant so heights merge normally.
• SelectTrigger defaulted to w-fit while Input defaults to w-full, forcing every call site to re-add w-full. Default is now w-full.

Also introduces Field, a label/control/hint/error stack primitive, so future spacing changes live in one place instead of every form. Form migration to Field is PR2.

[marekdano]

The core fixes are solid.

The new Field primitive has a11y gaps:

1. [High] field.tsx:~116 — aria-invalid/aria-describedby silently no-op on composite children (e.g. Select) Wrapping <Select><SelectTrigger id="type">...</SelectTrigger></Select> in <Field id="type" error="Required"> clones the aria props onto <Select>, but Select (select.tsx:29-31) just spreads ...props onto SelectPrimitive.Root, a Radix context provider that renders no DOM element — the actual SelectTrigger never receives them. Verified empirically: trigger.getAttribute('aria-invalid') and aria-describedby both came back null. Since select.tsx is touched in this same PR and select-based fields are the next thing scheduled to migrate onto Field ("PR2"), this would silently drop error-state a11y wiring and styling (aria-invalid:border-destructive) on every select field.

2. [High] field.tsx:~116 — aria-invalid/aria-describedby get cloned onto every child, not just the form control <Field id="name" error="Required"><Input id="name"/><Button>Clear</Button></Field> (a field with a trailing action) marks the Clear button as aria-invalid="true" and aria-describedby="name-error" too. Verified empirically: both Input and the sibling Button received the same attributes. Screen readers will announce the Clear button as invalid and "described by: Required," which is wrong.

3. [Medium] field.tsx:~124 — hint text has no id and is never wired into aria-describedby <Field id="name" label="Name" hint="Must be unique"><Input id="name"/></Field> renders the hint paragraph with no id, and the aria-describedby injected onto the Input only ever comes from errorId (undefined here) or the child's pre-existing aria-describedby — never the hint. A screen reader user tabbing into the input gets no indication the hint exists, defeating the component's stated purpose.

4. [Low] field.tsx:~112 — labelProps spreads after the auto-derived htmlFor, silently overriding it <Field id="email" label="Email" labelProps={{ htmlFor: "wrong-id" }}><Input id="email"/></Field> produces <label for="wrong-id"> that no longer targets the actual input, breaking click-to-focus and the screen-reader label association, with no warning.

[gcgoncalves]

@marekdano Addressed.

[marekdano]

🔴 src/index.css:256 — line-height fix likely doesn't apply in production

The new line-height: 1 rule for [data-slot="label"] is wrapped in @layer components, but Tailwind's .text-sm utility lives in @layer utilities, declared later. Under CSS Cascade Layers, a rule in a later layer beats a rule in an earlier layer for the same property on the same element, regardless of selector specificity — so .text-sm's line-height (≈1.4286) still wins over this new rule. Since Label's CVA base class always includes text-sm, every rendered <Label> keeps the old line-height — this is the exact bug the PR claims to fix, and it still exists.

Verified against an actual vite build of this branch — the compiled CSS shows:

@layer components{[data-slot=label]{line-height:1}}
@layer utilities{...}.text-sm{font-size:var(--text-sm);line-height:var(--tw-leading,var(--text-sm--line-height))}

Vitest/jsdom tests pass because jsdom doesn't apply real cascade-layer semantics, so this isn't caught by CI.

The fix: declare the rule unlayered instead of inside @layer components, matching the pattern already documented a few lines above it in src/index.css ("Unlayered on purpose: Tailwind emits .overflow-y-auto into @layer utilities, which these rules have to outrank at equal specificity").

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🟡 src/components/ui/field.tsx:28 — dual child API adds unnecessary surface (minor)

Field supports both a cloned single ReactElement child and a render-prop function. The render-prop form alone already covers every call site (including plain Input/Textarea), so the cloneElement branch is extra surface to maintain — it has subtly different prop-merging behavior (id always overrides, but aria-invalid/aria-describedby only fall back to the child's own value) and requires two Record<string, unknown> casts.

Suggest dropping the cloneElement branch and always using the render-prop form ({(p) => <Input {...p} />}) — removes ~10 lines and one path callers could pick incorrectly.