Phase 1: Infrastructure setup - Install next-intl and create basic i18n structure

- Install next-intl package
- Create LanguageProvider hook with localStorage persistence
- Create IntlProvider component for next-intl integration
- Create Providers wrapper component
- Update layout.tsx to include providers and set default lang to 'es'
- Create initial translation files (common.json) for es, en, ca
- Fix pre-existing TypeScript errors in various pages

All tests passing, build successful.

frontend/app/admin/price-history/page.tsx

@@ -63,7 +63,7 @@ export default function AdminPriceHistoryPage() {
         <div style={styles.tableHeader}>
           <h2 style={styles.tableTitle}>Bitcoin Price History</h2>
           <div style={styles.headerActions}>
-            <span style={styles.totalCount}>{records.length} records</span>
+            <span style={styles.totalCount}>{records?.length ?? 0} records</span>
             <button onClick={fetchRecords} style={styles.refreshBtn} disabled={isLoadingData}>
               Refresh
             </button>
@@ -98,7 +98,7 @@ export default function AdminPriceHistoryPage() {
                   </td>
                 </tr>
               )}
-              {!error && !isLoadingData && records.length === 0 && (
+              {!error && !isLoadingData && (records?.length ?? 0) === 0 && (
                 <tr>
                   <td colSpan={4} style={styles.emptyRow}>
                     No price records yet. Click &quot;Fetch Now&quot; to get the current price.
@@ -107,7 +107,7 @@ export default function AdminPriceHistoryPage() {
               )}
               {!error &&
                 !isLoadingData &&
-                records.map((record) => (
+                (records ?? []).map((record) => (
                   <tr key={record.id} style={styles.tr}>
                     <td style={styles.td}>{record.source}</td>
                     <td style={styles.td}>{record.pair}</td>