Add settings management with Bundesland selection and holiday calculations

db/schema.sql

@@ -6,3 +6,9 @@ CREATE TABLE IF NOT EXISTS entries (
   pause_minutes INTEGER NOT NULL DEFAULT 0,
   location TEXT DEFAULT 'office' CHECK(location IN ('office', 'home'))
 );
+
+CREATE TABLE IF NOT EXISTS settings (
+  key TEXT PRIMARY KEY,
+  value TEXT NOT NULL,
+  updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);