Form primitives, keyboard-operable comboboxes, dialog focus, route states

Accessibility work on the UI layer, all of it rooted in one structural gap:
there were no form primitives, so every field was hand-assembled and every
field got the same details wrong.

Form primitives
- components/ui/Field.tsx (Field/TextField/SelectField/TextareaField) and
  Button.tsx. Field generates the control id with useId and derives htmlFor
  from it, which is what makes the association impossible to omit rather
  than merely conventional.
- 92 labels existed, 4 used htmlFor, and no input carried an id at all: a
  screen reader announced an unnamed edit box and clicking a label focused
  nothing. Now every label resolves to its control (0 unassociated), and the
  input class chain that appeared verbatim 85 times appears zero times.
- Field also takes a render prop, so Lookup, CountryPicker and Picklist get
  the same wiring instead of a second, partial solution.
- SearchInput replaces three hand-rolled copies of the icon-in-a-box search
  whose input had only a placeholder — not a label — and killed its own
  focus ring with outline-none and nothing in its place.
- Toggle groups (workdays, reorg change type) became fieldsets with
  aria-pressed; colour alone was carrying the selected state.

Comboboxes
- Lookup and CountryPicker were text inputs with a div of clickable buttons
  underneath: typeable, but no keyboard path to a result and nothing telling
  a screen reader a list had appeared. Both now carry role=combobox,
  aria-expanded/controls/activedescendant and listbox semantics, with arrow
  keys, Enter and Escape. Escape stops propagation, or it would close the
  surrounding dialog along with the dropdown.

Dialogs
- useDialogFocus centralises what Modal and SlideOver each owed the
  keyboard and neither provided beyond Escape: focus into the dialog on
  open, Tab and Shift+Tab cycling within it, focus restored to the trigger
  on close.
- SlideOver stays mounted for its transition, and aria-hidden does not
  remove anything from the tab order — so every closed panel was leaving
  invisible tab stops at the end of the page. `inert` fixes that.

Route states
- loading.tsx, error.tsx, not-found.tsx and global-error.tsx. Every page in
  the (app) group is server-rendered per request, so without loading.tsx a
  navigation showed nothing at all until the server answered, and a render
  error dropped the user on Next's own screen with no way back.

Tests
- 22 component tests (vitest jsdom project). Two of them found limits of the
  environment rather than of the code: jsdom implements neither `inert` nor
  scrollIntoView, so the inert test asserts the attribute and the missing
  scrollIntoView — which was taking the whole render down from inside an
  effect — is stubbed in the setup file.

components/ui/Button.tsx

@@ -0,0 +1,74 @@
+"use client";
+
+import type { ButtonHTMLAttributes, ReactNode } from "react";
+
+// The primary-button class chain was written out by hand at a dozen call
+// sites and the ghost/secondary ones at many more, each drifting slightly in
+// padding and hover colour. Collecting them here also gives every button a
+// visible keyboard focus ring, which none of them had.
+
+const BASE =
+  "inline-flex items-center justify-center gap-1.5 rounded font-semibold transition-colors " +
+  "focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-brand-500 " +
+  "disabled:cursor-not-allowed disabled:opacity-50";
+
+const VARIANTS = {
+  primary: "bg-brand-500 text-white hover:bg-brand-600",
+  secondary: "border border-border bg-white text-ink-body hover:bg-surface",
+  ghost: "text-ink-body hover:bg-surface",
+  danger: "bg-danger-solid text-white hover:brightness-110",
+  // Square icon-only button; pair with an aria-label.
+  icon: "text-ink-muted hover:bg-surface hover:text-ink",
+} as const;
+
+const SIZES = {
+  sm: "px-3 py-1.5 text-xs",
+  md: "px-4 py-2 text-sm",
+  // Meets the 44px touch target without looking oversized on desktop.
+  icon: "p-2",
+} as const;
+
+/**
+ * For anchors that read as buttons (download links, cross-page actions). A
+ * real <a> keeps middle-click and "open in new tab" working, which a button
+ * with an onClick would throw away — so those stay anchors and borrow the
+ * styling instead.
+ */
+export const LINK_BUTTON_CLASS = `${BASE} ${VARIANTS.secondary} ${SIZES.sm}`;
+
+type ButtonProps = Omit<ButtonHTMLAttributes<HTMLButtonElement>, "className"> & {
+  variant?: keyof typeof VARIANTS;
+  size?: keyof typeof SIZES;
+  /** Disables and shows the busy label; use for in-flight server actions. */
+  pending?: boolean;
+  pendingLabel?: string;
+  fullWidth?: boolean;
+  className?: string;
+  children?: ReactNode;
+};
+
+export function Button({
+  variant = "primary",
+  size,
+  pending = false,
+  pendingLabel,
+  fullWidth,
+  disabled,
+  className = "",
+  children,
+  type = "button",
+  ...rest
+}: ButtonProps) {
+  const resolvedSize = size ?? (variant === "icon" ? "icon" : "md");
+  return (
+    <button
+      {...rest}
+      type={type}
+      disabled={disabled || pending}
+      aria-busy={pending || undefined}
+      className={`${BASE} ${VARIANTS[variant]} ${SIZES[resolvedSize]} ${fullWidth ? "w-full" : ""} ${className}`}
+    >
+      {pending && pendingLabel ? pendingLabel : children}
+    </button>
+  );
+}