feat: add expense date and CSV export to budget

- New expense_date column on budget items (DB migration #42)
- Date column in budget table with custom date picker
- CSV export button with BOM, semicolon separator, localized dates,
  currency in header, per-person/day calculations
- CustomDatePicker compact/borderless modes for inline table use
- i18n keys for all 12 languages

server/src/db/migrations.ts

@@ -436,6 +436,9 @@ function runMigrations(db: Database.Database): void {
     () => {
       try { db.exec('ALTER TABLE trips ADD COLUMN reminder_days INTEGER DEFAULT 3'); } catch {}
     },
+    () => {
+      try { db.exec('ALTER TABLE budget_items ADD COLUMN expense_date TEXT DEFAULT NULL'); } catch {}
+    },
   ];
 
   if (currentVersion < migrations.length) {