diff --git a/web/src/components/ui/tooltip.test.tsx b/web/src/components/ui/tooltip.test.tsx
new file mode 100644
index 0000000..b2e6e80
--- /dev/null
+++ b/web/src/components/ui/tooltip.test.tsx
@@ -0,0 +1,83 @@
+import { describe, it, expect } from "vitest";
+import { render, screen, within } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { Tooltip as TooltipPrimitive } from "@base-ui/react/tooltip";
+
+import {
+  Tooltip,
+  TooltipProvider,
+  TooltipRoot,
+  TooltipTrigger,
+  TooltipPositioner,
+  TooltipPopup,
+} from "./tooltip";
+
+describe("Tooltip", () => {
+  it("shows its content in a portal on hover", async () => {
+    const user = userEvent.setup();
+
+    render(
+      <Tooltip content="Objects">
+        <button type="button">Objects nav</button>
+      </Tooltip>,
+    );
+
+    const trigger = screen.getByRole("button", { name: "Objects nav" });
+    const body = within(document.body);
+
+    expect(body.queryByText("Objects")).toBeNull();
+
+    await user.hover(trigger);
+
+    expect(await body.findByText("Objects")).toBeVisible();
+  });
+
+  it("shows its content on keyboard focus", async () => {
+    const user = userEvent.setup();
+
+    render(
+      <Tooltip content="Help text">
+        <button type="button">Focusable</button>
+      </Tooltip>,
+    );
+
+    const body = within(document.body);
+
+    await user.tab();
+
+    expect(await body.findByText("Help text")).toBeVisible();
+  });
+
+  it("delegates the trigger element via render (keeps the host tag)", () => {
+    render(
+      <Tooltip content="Tip">
+        <a href="/objects">Go</a>
+      </Tooltip>,
+    );
+
+    const link = screen.getByRole("link", { name: "Go" });
+    expect(link).toHaveAttribute("href", "/objects");
+  });
+
+  it("composes from the raw parts", async () => {
+    const user = userEvent.setup();
+
+    render(
+      <TooltipProvider delay={0}>
+        <TooltipRoot>
+          <TooltipTrigger render={<button type="button">Raw</button>} />
+          <TooltipPrimitive.Portal>
+            <TooltipPositioner side="bottom">
+              <TooltipPopup>Raw tip</TooltipPopup>
+            </TooltipPositioner>
+          </TooltipPrimitive.Portal>
+        </TooltipRoot>
+      </TooltipProvider>,
+    );
+
+    await user.hover(screen.getByRole("button", { name: "Raw" }));
+
+    const body = within(document.body);
+    expect(await body.findByText("Raw tip")).toBeVisible();
+  });
+});
diff --git a/web/src/components/ui/tooltip.tsx b/web/src/components/ui/tooltip.tsx
new file mode 100644
index 0000000..53b4512
--- /dev/null
+++ b/web/src/components/ui/tooltip.tsx
@@ -0,0 +1,82 @@
+import type { ReactElement, ReactNode } from "react";
+import { Tooltip as TooltipPrimitive } from "@base-ui/react/tooltip";
+
+import { cn } from "@/lib/utils";
+
+type Side = NonNullable<TooltipPrimitive.Positioner.Props["side"]>;
+
+function TooltipProvider({ ...props }: TooltipPrimitive.Provider.Props) {
+  return <TooltipPrimitive.Provider {...props} />;
+}
+
+function TooltipRoot({ ...props }: TooltipPrimitive.Root.Props) {
+  return <TooltipPrimitive.Root data-slot="tooltip" {...props} />;
+}
+
+function TooltipTrigger({ ...props }: TooltipPrimitive.Trigger.Props) {
+  return <TooltipPrimitive.Trigger data-slot="tooltip-trigger" {...props} />;
+}
+
+function TooltipPopup({ className, ...props }: TooltipPrimitive.Popup.Props) {
+  return (
+    <TooltipPrimitive.Popup
+      data-slot="tooltip-popup"
+      className={cn(
+        "rounded border bg-white px-2 py-1 text-sm shadow-md",
+        className,
+      )}
+      {...props}
+    />
+  );
+}
+
+function TooltipPositioner({ className, ...props }: TooltipPrimitive.Positioner.Props) {
+  return (
+    <TooltipPrimitive.Positioner
+      data-slot="tooltip-positioner"
+      className={cn("z-50", className)}
+      {...props}
+    />
+  );
+}
+
+export type TooltipProps = {
+  /** Text shown in the tooltip popup. */
+  content: ReactNode;
+  /** The element the tooltip is attached to. Rendered as the trigger. */
+  children: ReactElement;
+  /** Which side of the trigger to place the popup. Defaults to `"right"`. */
+  side?: Side;
+  /** Pixel gap between the trigger and the popup. */
+  sideOffset?: number;
+};
+
+/**
+ * Standalone tooltip: wraps its own `Provider`, so it can be dropped in
+ * anywhere without an ancestor provider. `children` is delegated to the
+ * Base UI trigger via `render`, so the underlying element keeps its own
+ * tag/handlers (e.g. a `NavLink` for the collapsed sidebar).
+ */
+function Tooltip({ content, children, side = "right", sideOffset = 6 }: TooltipProps) {
+  return (
+    <TooltipProvider>
+      <TooltipRoot>
+        <TooltipTrigger render={children} />
+        <TooltipPrimitive.Portal>
+          <TooltipPositioner side={side} sideOffset={sideOffset}>
+            <TooltipPopup>{content}</TooltipPopup>
+          </TooltipPositioner>
+        </TooltipPrimitive.Portal>
+      </TooltipRoot>
+    </TooltipProvider>
+  );
+}
+
+export {
+  Tooltip,
+  TooltipProvider,
+  TooltipRoot,
+  TooltipTrigger,
+  TooltipPositioner,
+  TooltipPopup,
+};
diff --git a/web/src/lib/use-media-query.ts b/web/src/lib/use-media-query.ts
new file mode 100644
index 0000000..c918347
--- /dev/null
+++ b/web/src/lib/use-media-query.ts
@@ -0,0 +1,20 @@
+import { useEffect, useState } from "react";
+
+/** SSR-safe `matchMedia` subscription; `true` while `query` matches. */
+export function useMediaQuery(query: string): boolean {
+  const [matches, setMatches] = useState(() =>
+    typeof window !== "undefined" ? window.matchMedia(query).matches : false,
+  );
+
+  useEffect(() => {
+    const mql = window.matchMedia(query);
+    const onChange = () => setMatches(mql.matches);
+
+    onChange();
+    mql.addEventListener("change", onChange);
+
+    return () => mql.removeEventListener("change", onChange);
+  }, [query]);
+
+  return matches;
+}