fix: repair test environment and four cross-package bugs

[stooit]

Summary

Makes the full suite pass (bun test → 13 pass / 0 fail) and clears all tsc --noEmit errors (exit 0). Fixes two test-environment/config gaps and four source bugs spanning all three packages. No test files were modified and no dependencies were added.

Changes

Config (unblocked 8 of the failures)

• bunfig.toml — [test].environment = "happy-dom" is Vitest syntax that Bun silently ignores, so tests ran with no DOM (document is not defined). Replaced with preload of the existing happy-dom setup.ts, so bare bun test now has DOM globals.
• tsconfig.json — added "types": ["bun-types"]. The package is installed but not under @types, so TS never auto-included it, causing Cannot find module 'bun:test'.

Source bugs

• apps/web/src/lib/api.ts — useThrottle → useDebounce. The hook was renamed in @e2e/utils (only useDebounce is exported; the implementation is a trailing debounce, and the consumer is a search input, so debounce is the correct primitive). Merged the duplicate @e2e/utils imports.
• packages/ui/.../Button.tsx — icon-only buttons now forward an aria-label (explicit label → string children → fallback), with a dev-only warning when unlabelled. Text buttons receive undefined so React omits the attribute and their accessible name still derives from visible text. [WCAG 4.1.2, 2.5.3]
• packages/ui/.../DataTable.tsx — sort handler uses the functional setSortDir(prev => …) form, removing a stale-closure/batched-render hazard.
• packages/utils/.../date.ts — formatDate emits day-first en-AU output without a leading-zero day, using formatToParts to preserve a 4-digit year (avoids dateStyle: "short" truncating to 1/3/24).

Verification

• bun test → 13 pass / 0 fail (each package also passes standalone).
• bunx tsc --noEmit → exit 0.
• Each change was independently confirmed load-bearing (reverting it reintroduces a failure), except the DataTable change which is safe hardening.

Assumptions / follow-ups (out of scope)

• Date format is a hybrid D/MM/YYYY (unpadded day, padded month) — the least-bad way to satisfy the existing /^1/ assertion without truncating the year. A follow-up should align on one canonical format and adjust the test.
• "Button" aria-label fallback satisfies WCAG's letter but conveys little; the dev warning steers callers to pass a meaningful label. A compile-time requirement wasn't possible because a test renders an unlabelled icon button.
• Pre-existing timezone fragility in formatDate (ambient TZ) is retained; it passes on UTC/AU runners but would flake at UTC+13/+14.
• Root package.json pins bun-types to latest; now load-bearing for tsc, worth pinning later.