Remove duplicate utility code via shared formatting module

Consolidate MONTH_NAMES, PALETTE, formatCurrency, formatCurrencyShort,
ordinal, and fmt into client/src/utils/formatting.js. Consolidate
CONFIG_KEYS into server/src/constants.js. All consumer files updated
to import from these shared modules. PaycheckView re-exports formatCurrency,
ordinal, and formatPayDate for backward compatibility with existing tests.

Nightshift-Task: dead-code
Nightshift-Ref: https://github.com/marcus/nightshift

server/src/constants.js

@@ -0,0 +1,12 @@
+'use strict';
+
+const CONFIG_KEYS = [
+  'paycheck1_day',
+  'paycheck2_day',
+  'paycheck1_gross',
+  'paycheck1_net',
+  'paycheck2_gross',
+  'paycheck2_net',
+];
+
+module.exports = { CONFIG_KEYS };