refactor: extract shared locale, date helpers, and components (H6)

~300 lines of code that were independently duplicated in Dashboard.tsx
and Comparison.tsx are now in shared modules:

- src/lib/locale.ts — LC interface, EN and AR language configs (merged
  fields from both pages into one unified interface)
- src/lib/dateHelpers.ts — MONTH_KEYS, isLeap, makePresets, guessPreset,
  periodNameL, dateRangeTextL, currentMonth, shiftYear
- src/components/shared/PeriodPicker.tsx — InlinePicker + PeriodHero
- src/components/shared/AltMultiSelect.tsx — AltMultiSelect
- src/components/shared/MetricCard.tsx — MetricCard

Dashboard.tsx and Comparison.tsx now import from these shared modules.
Zero behavioral changes — all props, ARIA, and render output unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/shared/MetricCard.tsx

@@ -0,0 +1,23 @@
+import React from 'react';
+import { formatCurrency, formatNumber } from '../../services/dataService';
+
+// ─── metric card ──────────────────────────────────────────────────
+export default function MetricCard({ title, curr, prev, isCurrency, newLabel }: {
+  title: string; curr: number; prev: number; isCurrency?: boolean; newLabel?: string;
+}) {
+  const fmt = (n: number) => isCurrency ? formatCurrency(n) : formatNumber(n);
+  const change = prev === 0 ? (curr > 0 ? Infinity : 0) : ((curr - prev) / prev * 100);
+  const isPos = change > 0, isNeg = change < 0;
+  return (
+    <div className="alt-metric">
+      <p className="alt-metric-title">{title}</p>
+      <div className="alt-metric-value">{fmt(curr)}</div>
+      <div className="alt-metric-footer">
+        {isFinite(change)
+          ? <span className={`alt-change ${isPos ? 'alt-change--up' : isNeg ? 'alt-change--down' : 'alt-change--flat'}`}>{isPos ? '▲' : isNeg ? '▼' : '—'} {Math.abs(change).toFixed(1)}%</span>
+          : <span className="alt-change alt-change--up">{newLabel ?? 'New'}</span>}
+        <span className="alt-metric-prev">{fmt(prev)}</span>
+      </div>
+    </div>
+  );
+}