docs(adr): add 0001-base-ui-over-radix

docs/adr/0001-base-ui-over-radix.md

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+# ADR-0001: Use @base-ui/react instead of Radix UI
+
+## Status
+
+accepted
+
+## Context
+
+shadcn/ui ships visual components that are typically built on Radix UI primitives. When we bootstrapped Scrum4Me with shadcn, the component wrappers in `components/ui/` were adapted to use `@base-ui/react` instead of the Radix packages. `@base-ui/react` exposes the same accessibility primitives but uses a `render` prop for composition instead of Radix's `asChild` pattern. Attempting to use `asChild` in our TypeScript-strict setup produced type errors because the prop is not declared in `@base-ui/react`'s API surface.
+
+## Decision
+
+We use `@base-ui/react` exclusively. No Radix UI package (`@radix-ui/*`) is imported anywhere in the codebase. Composition always uses the `render` prop:
+
+```tsx
+// ✅ correct
+<TooltipTrigger render={<button />}>…</TooltipTrigger>
+
+// ❌ wrong — asChild does not exist on @base-ui/react primitives
+<TooltipTrigger asChild><button>…</button></TooltipTrigger>
+```
+
+## Consequences
+
+### Positive
+
+- TypeScript stays clean; no `any` casts or `asChild` workarounds.
+- `@base-ui/react` is actively maintained by the MUI team with React 19 support.
+- Composition pattern is explicit and grep-friendly.
+
+### Negative
+
+- AI agents trained on Radix-based shadcn documentation will default to `asChild` — they must be reminded of the `render`-prop pattern (this ADR exists for that reason).
+- shadcn CLI-generated components may need manual adjustment when installed.