Daten aendern: effective date, searchable UN country pickers, 2 more bugfixes

Feature requests from live use:
- "Daten aendern" was missing a "Wirksam ab" field (unlike Versetzen/
  Befoerdern/Karenz, which all have one) - every change was silently
  logged with today's date. Added the field, threaded through
  change_employee_data (defaults to today if omitted).
- Staatsbuergerschaft and Wohnland now use a searchable picker
  (components/ui/CountryPicker) over the full 193-country UN member
  state list (lib/countries.ts) instead of the original ~9/5-value
  picklists. Dropped the now-too-narrow CHECK constraints
  (supabase/schema_2.sql) since the app is the source of truth for
  valid values, same approach used elsewhere for large open-ended
  pickers.

Two more real bugs found via live testing of the above (both in
change_employee_data, supabase/functions.sql + functions_4.sql):
1. `text[] || 'literal'` is ambiguous in Postgres - it can resolve to
   the array||array overload and try to parse the plain word as array
   syntax ('{...}'), failing with "malformed array literal". Hit on
   every single field-diff line the moment a user actually changed
   something (Staatsbuergerschaft first, then Beschaeftigungsausmass
   confirmed the same root cause). Fixed everywhere by switching to the
   unambiguous array_append() function.
2. The contract_end_date diff-check cast an empty string straight to
   date ("invalid input syntax for type date: ''") instead of using the
   same nullif(...,'')::date guard the UPDATE line below it already had.

Verified live end-to-end after both fixes: changed Staatsbuergerschaft
to Brasilien with a backdated effective date, save succeeded, Stammdaten
tab reflects it, and employee_history got the correct event_date
("2026-07-01") and description ("Geänderte Felder: Staatsbürgerschaft,
wirksam ab 2026-07-01"). Reverted the test employee's data back
afterward; seed data is clean again.

components/ui/CountryPicker.tsx

@@ -0,0 +1,68 @@
+"use client";
+
+import { useEffect, useRef, useState } from "react";
+
+type CountryPickerProps = {
+  value: string;
+  onChange: (value: string) => void;
+  countries: string[];
+  placeholder?: string;
+};
+
+// Searchable text combobox constrained to a fixed list (UN member states)
+// rather than a Lookup-style "selected card" — a country is a single text
+// value, not an object with its own detail fields.
+export function CountryPicker({ value, onChange, countries, placeholder = "Land suchen…" }: CountryPickerProps) {
+  const [query, setQuery] = useState(value);
+  const [open, setOpen] = useState(false);
+  const containerRef = useRef<HTMLDivElement>(null);
+
+  useEffect(() => setQuery(value), [value]);
+
+  useEffect(() => {
+    function onClickOutside(e: MouseEvent) {
+      if (containerRef.current && !containerRef.current.contains(e.target as Node)) {
+        setOpen(false);
+        setQuery(value);
+      }
+    }
+    document.addEventListener("mousedown", onClickOutside);
+    return () => document.removeEventListener("mousedown", onClickOutside);
+  }, [value]);
+
+  const filtered = query.trim() ? countries.filter((c) => c.toLowerCase().includes(query.trim().toLowerCase())) : countries;
+
+  return (
+    <div ref={containerRef} className="relative">
+      <input
+        value={query}
+        onChange={(e) => {
+          setQuery(e.target.value);
+          setOpen(true);
+        }}
+        onFocus={() => setOpen(true)}
+        placeholder={placeholder}
+        className="w-full rounded border border-border px-3 py-2 text-sm"
+      />
+      {open && (
+        <div className="absolute z-20 mt-1 max-h-56 w-full overflow-y-auto rounded border border-border bg-white shadow-lg">
+          {filtered.length === 0 && <div className="px-3 py-2 text-sm text-ink-muted">Keine Treffer</div>}
+          {filtered.slice(0, 50).map((c) => (
+            <button
+              key={c}
+              type="button"
+              onClick={() => {
+                onChange(c);
+                setQuery(c);
+                setOpen(false);
+              }}
+              className="block w-full px-3 py-2 text-left text-sm hover:bg-surface"
+            >
+              {c}
+            </button>
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}