fix(frontend): normalize empty ?claim= to null; preserve sibling params

- readClaimId: URLSearchParams.get returns '' for ?claim=, not null.
  Normalize to null so the contract matches the JSDoc and consumers
  can treat empty as absent.

- Tests: regression for ?claim= → null; open/close preserves other
  query params (?foo=bar survives open/close unchanged).

- Tighten pushStateMock/replaceStateMock typing to vi.fn<() => void>()
  per code-review nit (drops the verbose ReturnType<typeof vi.fn>).

src/hooks/useDrawerUrlState.ts

@@ -11,7 +11,11 @@ import { useCallback, useEffect, useState } from "react";
  */
 function readClaimId(): string | null {
   const params = new URLSearchParams(window.location.search);
-  return params.get("claim");
+  // `URLSearchParams.get` returns `""` for `?claim=` (param present but
+  // empty) — we normalize that to `null` so the "absent" and "empty"
+  // cases are indistinguishable to consumers, matching the JSDoc.
+  const value = params.get("claim");
+  return value === "" ? null : value;
 }
 
 /**