feat(shared): bidirectional split balance, credit direction, and multi-user view

- Rewrite participant balance to UNION both directions (they owe me + I owe them)
- Credits/refunds subtract from owed amount for correct net balance
- Allow secondary users to see transactions split with them
- Add participant balance cards with colour-coded owe direction
- Add inline AddParticipantForm with name + optional email

src/app/shared/page.tsx

@@ -5,6 +5,7 @@ import {
   useSharedTransactions,
   useParticipantBalances,
   useSettleSplits,
+  useCreateParticipant,
 } from "@/lib/hooks";
 import type { SharedTransactionRow } from "@/lib/queries";
 
@@ -12,8 +13,67 @@ function formatDate(d: string) {
   return new Date(d).toLocaleDateString("en-AU", { day: "numeric", month: "short", year: "numeric" });
 }
 
-function formatAmount(n: number) {
+const SPEND_TYPES = new Set(["debit", "fee", "interest"]);
-  return `$${Number(n).toFixed(2)}`;
+
+function formatAmount(n: number, type?: string) {
+  const formatted = `$${Number(n).toFixed(2)}`;
+  return type && !SPEND_TYPES.has(type) ? `+${formatted}` : formatted;
+}
+
+function AddParticipantForm({ onDone }: { onDone: () => void }) {
+  const [name, setName] = useState("");
+  const [email, setEmail] = useState("");
+  const [error, setError] = useState("");
+  const create = useCreateParticipant();
+
+  async function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+    setError("");
+    if (!name.trim()) { setError("Name is required"); return; }
+    try {
+      await create.mutateAsync({ name: name.trim(), email: email.trim() || undefined });
+      onDone();
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Failed to create");
+    }
+  }
+
+  return (
+    <form onSubmit={handleSubmit} className="bg-zinc-900 border border-zinc-700 rounded-xl p-4 space-y-3">
+      <p className="text-sm font-medium">Add Participant</p>
+      <div className="flex gap-2">
+        <input
+          type="text"
+          placeholder="Name"
+          value={name}
+          onChange={(e) => setName(e.target.value)}
+          className="flex-1 bg-zinc-800 border border-zinc-700 rounded-lg px-3 py-1.5 text-sm focus:outline-none focus:border-zinc-500"
+        />
+        <input
+          type="email"
+          placeholder="Email (optional)"
+          value={email}
+          onChange={(e) => setEmail(e.target.value)}
+          className="flex-1 bg-zinc-800 border border-zinc-700 rounded-lg px-3 py-1.5 text-sm focus:outline-none focus:border-zinc-500"
+        />
+        <button
+          type="submit"
+          disabled={create.isPending}
+          className="px-3 py-1.5 bg-blue-600 hover:bg-blue-700 disabled:opacity-50 text-white rounded-lg text-sm font-medium"
+        >
+          {create.isPending ? "Adding..." : "Add"}
+        </button>
+        <button
+          type="button"
+          onClick={onDone}
+          className="px-3 py-1.5 bg-zinc-800 hover:bg-zinc-700 text-zinc-400 rounded-lg text-sm"
+        >
+          Cancel
+        </button>
+      </div>
+      {error && <p className="text-red-400 text-xs">{error}</p>}
+    </form>
+  );
 }
 
 export default function SharedPage() {
@@ -21,6 +81,7 @@ export default function SharedPage() {
   const { data: balances = [], isLoading: balLoading } = useParticipantBalances();
   const settle = useSettleSplits();
   const [settling, setSettling] = useState<number | null>(null);
+  const [addingParticipant, setAddingParticipant] = useState(false);
 
   async function handleSettleParticipant(participantId: number) {
     setSettling(participantId);
@@ -28,40 +89,60 @@ export default function SharedPage() {
     setSettling(null);
   }
 
-  const others = balances.filter((b) => b.name !== "Me");
-
   return (
     <div className="space-y-6">
-      <h2 className="text-xl font-semibold">Shared Expenses</h2>
+      <div className="flex items-center justify-between">
+        <h2 className="text-xl font-semibold">Shared Expenses</h2>
+        {!addingParticipant && (
+          <button
+            onClick={() => setAddingParticipant(true)}
+            className="text-sm px-3 py-1.5 bg-zinc-800 hover:bg-zinc-700 text-zinc-300 rounded-lg"
+          >
+            + Add Participant
+          </button>
+        )}
+      </div>
+
+      {addingParticipant && (
+        <AddParticipantForm onDone={() => setAddingParticipant(false)} />
+      )}
 
       {/* 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) => (
+          balances.map((b) => {
-            <div key={b.id} className="bg-zinc-900 border border-zinc-700 rounded-xl p-4">
+            const theyOweMe = b.total_owed > 0;
-              <div className="flex items-start justify-between mb-3">
+            const net = Math.abs(b.total_owed);
-                <div>
+            return (
-                  <p className="font-medium">{b.name}</p>
+              <div key={b.id} className="bg-zinc-900 border border-zinc-700 rounded-xl p-4">
-                  <p className="text-xs text-zinc-500">{b.unsettled_count} unsettled</p>
+                <div className="flex items-start justify-between mb-3">
-                </div>
+                  <div>
-                <div className="text-right">
+                    <p className="font-medium">{b.name}</p>
-                  <p className="text-lg font-semibold text-amber-400">{formatAmount(b.total_owed)}</p>
+                    <p className="text-xs text-zinc-500">
-                  <p className="text-xs text-zinc-500">owes you</p>
+                      {theyOweMe ? `${b.unsettled_count} unsettled` : "you owe"}
+                    </p>
+                  </div>
+                  <div className="text-right">
+                    <p className={`text-lg font-semibold ${theyOweMe ? "text-amber-400" : "text-blue-400"}`}>
+                      ${net.toFixed(2)}
+                    </p>
+                    <p className="text-xs text-zinc-500">{theyOweMe ? "owes you" : "you owe"}</p>
+                  </div>
                 </div>
+                {theyOweMe && 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>
-              {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>
 
@@ -100,8 +181,8 @@ export default function SharedPage() {
                       <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">
+                    <td className={`px-4 py-3 text-right font-medium tabular-nums ${SPEND_TYPES.has(tx.transaction_type) ? "" : "text-green-400"}`}>
-                      {formatAmount(tx.amount)}
+                      {formatAmount(tx.amount, tx.transaction_type)}
                     </td>
                     <td className="px-4 py-3">
                       <div className="flex flex-wrap gap-1">