Add /migrate: hand a legacy account over to coinos v3

coinos v3 links here with ?to=<the user's new payment address>&back=<url>.
The user is already signed in on this site, so no password or key has to
travel anywhere: we fetch an invoice from the v3 registrar for the
balance (less a small routing reserve), pay it, then rename the account
to <username>_v2 — which releases the username so v3's registrar will
let them claim it. Old account stays, renamed, and stops receiving.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Wvj4aoeMZwA7VUGdJP9Swi

2 files changed, 128 insertions, 0 deletions

diff --git a/src/routes/(app)/migrate/+page.server.ts b/src/routes/(app)/migrate/+page.server.ts
new file mode 100644
index 00000000..4529c14c
--- /dev/null
+++ b/src/routes/(app)/migrate/+page.server.ts
@@ -0,0 +1,10 @@
+import { redirect } from "@sveltejs/kit";
+
+export async function load({ parent, url }) {
+	const { user } = await parent();
+	if (!user) redirect(307, `/login?redirect=${encodeURIComponent(url.pathname + url.search)}`);
+	return {
+		to: url.searchParams.get("to") || "",
+		back: url.searchParams.get("back") || "",
+	};
+}
diff --git a/src/routes/(app)/migrate/+page.svelte b/src/routes/(app)/migrate/+page.svelte
new file mode 100644
index 00000000..4493709b
--- /dev/null
+++ b/src/routes/(app)/migrate/+page.svelte
@@ -0,0 +1,118 @@
+<script>
+  // Migrate a legacy coinos account to coinos v3.
+  //
+  // v3 sends the user here with ?to=<their new payment address>&back=<url>.
+  // They are already signed in on this site, so nothing has to handle their
+  // password: we sweep the balance over Lightning to the new wallet and then
+  // release the username, which lets v3 claim it on the new registrar.
+  import { post, success } from "$lib/utils";
+  import { goto } from "$app/navigation";
+
+  let { data } = $props();
+  let { user, to, back } = $derived(data);
+
+  let step = $state("intro"); // intro | working | done | error
+  let note = $state("");
+  let error = $state("");
+  let pin = $state("");
+  let sent = $state(0);
+
+  let balance = $derived(user?.balance || 0);
+  // Lightning routing takes a cut on the way out; leave a little behind so
+  // the sweep can't fail for being a few sats over.
+  let reserve = $derived(Math.max(10, Math.ceil(balance * 0.005)));
+  let amount = $derived(Math.max(0, balance - reserve));
+  let newName = $derived(`${user?.username}_v2`);
+
+  // Ask the v3 registrar for an invoice for this amount (LNURL-pay against
+  // the destination address). The registrar is CORS-open.
+  async function invoiceFor(address, sats) {
+    const [name, domain] = String(address).split("@");
+    if (!name || !domain) throw new Error("bad destination address");
+    const base = "https://names.coinos.io";
+    const p = await fetch(`${base}/.well-known/lnurlp/${name}?domain=${domain}`).then((r) => r.json());
+    if (!p?.callback) throw new Error(p?.reason || "destination not found");
+    const msat = sats * 1000;
+    if (msat < p.minSendable || msat > p.maxSendable) throw new Error("amount out of range for the destination");
+    const inv = await fetch(`${p.callback}${p.callback.includes("?") ? "&" : "?"}amount=${msat}`).then((r) => r.json());
+    if (!inv?.pr) throw new Error(inv?.reason || "could not get an invoice");
+    return inv.pr;
+  }
+
+  async function migrate() {
+    error = "";
+    step = "working";
+    try {
+      if (amount > 0) {
+        note = "Getting an invoice from your new wallet…";
+        const payreq = await invoiceFor(to, amount);
+        note = `Sending ${amount.toLocaleString()} sats…`;
+        await post("/payments", { payreq, ...(pin ? { pin } : {}) });
+        sent = amount;
+      }
+      note = "Releasing your username…";
+      await post("/user", { username: newName, ...(pin ? { pin } : {}) });
+      step = "done";
+      success("Migrated");
+    } catch (e) {
+      error = String(e && e.message ? e.message : e);
+      step = "error";
+    }
+  }
+
+  function goBack() {
+    const url = new URL(back || "https://v3.coinos.io/");
+    url.searchParams.set("migrated", user.username);
+    goto(url.toString(), { replaceState: true });
+  }
+</script>
+
+<div class="max-w-md mx-auto p-4 space-y-5">
+  <h1 class="text-2xl font-bold">Move your account to coinos v3</h1>
+
+  {#if !to}
+    <p class="text-red-600">
+      This page needs a destination. Start the migration from coinos v3.
+    </p>
+  {:else if step === "done"}
+    <div class="space-y-3">
+      <p class="text-xl">✓ All done!</p>
+      {#if sent}
+        <p>{sent.toLocaleString()} sats are on their way to your new wallet.</p>
+      {/if}
+      <p>
+        Your old username was released, so <strong>{user.username}</strong> can now be
+        yours on coinos v3.
+      </p>
+      <button class="btn btn-primary w-full" onclick={goBack}>Back to coinos v3</button>
+    </div>
+  {:else if step === "working"}
+    <p>{note}</p>
+    <progress class="progress w-full"></progress>
+  {:else}
+    <div class="space-y-3">
+      <p>
+        This moves your balance to your new wallet and frees up your username so you can
+        keep it there. Your old account stays, renamed to
+        <strong>{newName}</strong>, and stops receiving payments.
+      </p>
+      <div class="rounded-xl border p-3 space-y-1">
+        <div class="flex justify-between"><span>Balance</span><strong>{balance.toLocaleString()} sats</strong></div>
+        <div class="flex justify-between"><span>Sending</span><strong>{amount.toLocaleString()} sats</strong></div>
+        <div class="flex justify-between text-sm opacity-60"><span>Left for routing</span><span>{reserve.toLocaleString()} sats</span></div>
+        <div class="flex justify-between break-all gap-2"><span>To</span><span>{to}</span></div>
+      </div>
+      {#if user?.haspin}
+        <input class="input input-bordered w-full" type="password" inputmode="numeric"
+          placeholder="Your PIN" bind:value={pin} />
+      {/if}
+      {#if error}<p class="text-red-600">{error}</p>{/if}
+      <button class="btn btn-primary w-full" onclick={migrate} disabled={!to}>
+        Move {amount.toLocaleString()} sats and release {user.username}
+      </button>
+      <p class="text-sm opacity-60">
+        You can also do nothing — your old account keeps working as it is.
+      </p>
+    </div>
+  {/if}
+</div>