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.

lib/countries.ts

@@ -0,0 +1,199 @@
+// The 193 UN member states, German names, sorted alphabetically. Used for
+// Staatsbürgerschaft and Wohnland — both are searchable pickers over this
+// same list (see components/ui/Lookup + DatenAendernPanel).
+export const UN_COUNTRIES: string[] = [
+  "Afghanistan",
+  "Ägypten",
+  "Albanien",
+  "Algerien",
+  "Andorra",
+  "Angola",
+  "Antigua und Barbuda",
+  "Äquatorialguinea",
+  "Argentinien",
+  "Armenien",
+  "Aserbaidschan",
+  "Äthiopien",
+  "Australien",
+  "Bahamas",
+  "Bahrain",
+  "Bangladesch",
+  "Barbados",
+  "Belgien",
+  "Belize",
+  "Benin",
+  "Bhutan",
+  "Bolivien",
+  "Bosnien und Herzegowina",
+  "Botsuana",
+  "Brasilien",
+  "Brunei",
+  "Bulgarien",
+  "Burkina Faso",
+  "Burundi",
+  "Chile",
+  "China",
+  "Costa Rica",
+  "Côte d'Ivoire",
+  "Dänemark",
+  "Deutschland",
+  "Dominica",
+  "Dominikanische Republik",
+  "Dschibuti",
+  "Ecuador",
+  "El Salvador",
+  "Eritrea",
+  "Estland",
+  "Eswatini",
+  "Fidschi",
+  "Finnland",
+  "Frankreich",
+  "Gabun",
+  "Gambia",
+  "Georgien",
+  "Ghana",
+  "Grenada",
+  "Griechenland",
+  "Guatemala",
+  "Guinea",
+  "Guinea-Bissau",
+  "Guyana",
+  "Haiti",
+  "Honduras",
+  "Indien",
+  "Indonesien",
+  "Irak",
+  "Iran",
+  "Irland",
+  "Island",
+  "Israel",
+  "Italien",
+  "Jamaika",
+  "Japan",
+  "Jemen",
+  "Jordanien",
+  "Kambodscha",
+  "Kamerun",
+  "Kanada",
+  "Kap Verde",
+  "Kasachstan",
+  "Katar",
+  "Kenia",
+  "Kirgisistan",
+  "Kiribati",
+  "Kolumbien",
+  "Komoren",
+  "Kongo, Demokratische Republik",
+  "Kongo, Republik",
+  "Kosovo",
+  "Kroatien",
+  "Kuba",
+  "Kuwait",
+  "Laos",
+  "Lesotho",
+  "Lettland",
+  "Libanon",
+  "Liberia",
+  "Libyen",
+  "Liechtenstein",
+  "Litauen",
+  "Luxemburg",
+  "Madagaskar",
+  "Malawi",
+  "Malaysia",
+  "Malediven",
+  "Mali",
+  "Malta",
+  "Marokko",
+  "Marshallinseln",
+  "Mauretanien",
+  "Mauritius",
+  "Mexiko",
+  "Mikronesien",
+  "Moldau",
+  "Monaco",
+  "Mongolei",
+  "Montenegro",
+  "Mosambik",
+  "Myanmar",
+  "Namibia",
+  "Nauru",
+  "Nepal",
+  "Neuseeland",
+  "Nicaragua",
+  "Niederlande",
+  "Niger",
+  "Nigeria",
+  "Nordkorea",
+  "Nordmazedonien",
+  "Norwegen",
+  "Oman",
+  "Österreich",
+  "Osttimor",
+  "Pakistan",
+  "Palau",
+  "Panama",
+  "Papua-Neuguinea",
+  "Paraguay",
+  "Peru",
+  "Philippinen",
+  "Polen",
+  "Portugal",
+  "Ruanda",
+  "Rumänien",
+  "Russland",
+  "Salomonen",
+  "Sambia",
+  "Samoa",
+  "San Marino",
+  "São Tomé und Príncipe",
+  "Saudi-Arabien",
+  "Schweden",
+  "Schweiz",
+  "Senegal",
+  "Serbien",
+  "Seychellen",
+  "Sierra Leone",
+  "Simbabwe",
+  "Singapur",
+  "Slowakei",
+  "Slowenien",
+  "Somalia",
+  "Spanien",
+  "Sri Lanka",
+  "St. Kitts und Nevis",
+  "St. Lucia",
+  "St. Vincent und die Grenadinen",
+  "Südafrika",
+  "Sudan",
+  "Südkorea",
+  "Südsudan",
+  "Suriname",
+  "Syrien",
+  "Tadschikistan",
+  "Tansania",
+  "Thailand",
+  "Togo",
+  "Tonga",
+  "Trinidad und Tobago",
+  "Tschad",
+  "Tschechien",
+  "Tunesien",
+  "Türkei",
+  "Turkmenistan",
+  "Tuvalu",
+  "Uganda",
+  "Ukraine",
+  "Ungarn",
+  "Uruguay",
+  "Usbekistan",
+  "Vanuatu",
+  "Venezuela",
+  "Vereinigte Arabische Emirate",
+  "Vereinigte Staaten",
+  "Vereinigtes Königreich",
+  "Vietnam",
+  "Weißrussland",
+  "Zentralafrikanische Republik",
+  "Zypern",
+].sort((a, b) => a.localeCompare(b, "de"));