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.

supabase/schema_2.sql

@@ -0,0 +1,11 @@
+-- Addendum to supabase/schema.sql — run after that file.
+--
+-- Staatsbürgerschaft and Wohnland now use a searchable picker over the
+-- full UN member states list (193 countries, see lib/countries.ts)
+-- instead of the original ~9/5-value picklists. The old CHECK constraints
+-- would reject nearly all of those values, so they're dropped here. The
+-- app is the source of truth for valid values (same approach the rest of
+-- the app already relies on for large open-ended pickers); the columns
+-- stay plain text (nationality keeps its NOT NULL).
+alter table employees drop constraint if exists employees_nationality_check;
+alter table employees drop constraint if exists employees_address_country_check;