feat(finance): implement Shared Expenses page

Show split transactions with per-participant balance cards and settle buttons.

src/app/shared/page.tsx

@@ -1,8 +1,147 @@
+"use client";
+
+import { useState } from "react";
+import {
+  useSharedTransactions,
+  useParticipantBalances,
+  useSettleSplits,
+} from "@/lib/hooks";
+import type { SharedTransactionRow } from "@/lib/queries";
+
+function formatDate(d: string) {
+  return new Date(d).toLocaleDateString("en-AU", { day: "numeric", month: "short", year: "numeric" });
+}
+
+function formatAmount(n: number) {
+  return `$${Number(n).toFixed(2)}`;
+}
+
 export default function SharedPage() {
+  const { data: transactions = [], isLoading: txLoading } = useSharedTransactions();
+  const { data: balances = [], isLoading: balLoading } = useParticipantBalances();
+  const settle = useSettleSplits();
+  const [settling, setSettling] = useState<number | null>(null);
+
+  async function handleSettleParticipant(participantId: number) {
+    setSettling(participantId);
+    await settle.mutateAsync({ participant_id: participantId });
+    setSettling(null);
+  }
+
+  const others = balances.filter((b) => b.name !== "Me");
+
   return (
-    <div>
+    <div className="space-y-6">
-      <h2 className="text-xl font-semibold mb-4">Shared Expenses</h2>
+      <h2 className="text-xl font-semibold">Shared Expenses</h2>
-      <p className="text-zinc-500">Coming soon - track shared expenses and splits.</p>
+
+      {/* Balance summary */}
+      <div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">
+        {balLoading ? (
+          <p className="text-zinc-500 text-sm col-span-3">Loading balances...</p>
+        ) : (
+          others.map((b) => (
+            <div key={b.id} className="bg-zinc-900 border border-zinc-700 rounded-xl p-4">
+              <div className="flex items-start justify-between mb-3">
+                <div>
+                  <p className="font-medium">{b.name}</p>
+                  <p className="text-xs text-zinc-500">{b.unsettled_count} unsettled</p>
+                </div>
+                <div className="text-right">
+                  <p className="text-lg font-semibold text-amber-400">{formatAmount(b.total_owed)}</p>
+                  <p className="text-xs text-zinc-500">owes you</p>
+                </div>
+              </div>
+              {b.unsettled_count > 0 && (
+                <button
+                  onClick={() => handleSettleParticipant(b.id)}
+                  disabled={settling === b.id}
+                  className="w-full py-1.5 text-xs font-medium bg-emerald-700 hover:bg-emerald-600 text-white rounded-lg disabled:opacity-50"
+                >
+                  {settling === b.id ? "Settling..." : "Mark All Settled"}
+                </button>
+              )}
+            </div>
+          ))
+        )}
+      </div>
+
+      {/* Transaction list */}
+      <div className="bg-zinc-900 border border-zinc-700 rounded-xl overflow-hidden">
+        <div className="px-4 py-3 border-b border-zinc-800">
+          <h3 className="text-sm font-medium">Split Transactions</h3>
+        </div>
+        {txLoading ? (
+          <p className="text-zinc-500 text-sm px-4 py-6">Loading...</p>
+        ) : transactions.length === 0 ? (
+          <p className="text-zinc-500 text-sm px-4 py-6">
+            No split transactions yet. Use the Split button on any transaction.
+          </p>
+        ) : (
+          <table className="w-full text-sm">
+            <thead>
+              <tr className="border-b border-zinc-800">
+                <th className="text-left px-4 py-2 text-xs text-zinc-500 font-medium">Date</th>
+                <th className="text-left px-4 py-2 text-xs text-zinc-500 font-medium">Description</th>
+                <th className="text-right px-4 py-2 text-xs text-zinc-500 font-medium">Amount</th>
+                <th className="text-left px-4 py-2 text-xs text-zinc-500 font-medium">Splits</th>
+                <th className="px-4 py-2 text-xs text-zinc-500 font-medium">Action</th>
+              </tr>
+            </thead>
+            <tbody>
+              {(transactions as SharedTransactionRow[]).map((tx) => {
+                const splits = Array.isArray(tx.splits) ? tx.splits : [];
+                const unsettled = splits.filter((s) => !s.settled && s.name !== "Me");
+                return (
+                  <tr key={tx.id} className="border-b border-zinc-800/50 hover:bg-zinc-800/30">
+                    <td className="px-4 py-3 text-zinc-400 whitespace-nowrap">
+                      {formatDate(tx.transaction_date)}
+                    </td>
+                    <td className="px-4 py-3">
+                      <p className="font-medium truncate max-w-48">{tx.effective_merchant || tx.description}</p>
+                      <p className="text-xs text-zinc-500 truncate max-w-48">{tx.description}</p>
+                    </td>
+                    <td className="px-4 py-3 text-right font-medium tabular-nums">
+                      {formatAmount(tx.amount)}
+                    </td>
+                    <td className="px-4 py-3">
+                      <div className="flex flex-wrap gap-1">
+                        {splits.map((s) => (
+                          <span
+                            key={s.participant_id}
+                            className={`inline-flex items-center gap-1 px-2 py-0.5 rounded text-xs ${
+                              s.settled
+                                ? "bg-zinc-800 text-zinc-500"
+                                : "bg-amber-900/40 text-amber-300"
+                            }`}
+                          >
+                            {s.name} {s.share_percent}%
+                            {s.settled && <span className="text-emerald-500">✓</span>}
+                          </span>
+                        ))}
+                      </div>
+                    </td>
+                    <td className="px-4 py-3 text-center">
+                      {unsettled.length > 0 && (
+                        <button
+                          onClick={() =>
+                            settle.mutateAsync({
+                              split_ids: unsettled.map((s) => (s as unknown as { split_id: number }).split_id),
+                            })
+                          }
+                          disabled={settle.isPending}
+                          className="text-xs px-2 py-1 bg-emerald-800 hover:bg-emerald-700 text-white rounded disabled:opacity-50"
+                        >
+                          Settle
+                        </button>
+                      )}
+                    </td>
+                  </tr>
+                );
+              })}
+            </tbody>
+          </table>
+        )}
+      </div>
     </div>
   );
 }