fix(fx): the rate service moved, and a browser cannot follow its redirect

"Fetch rates now" failed on the deployed site with nothing but a CORS
error. api.frankfurter.app now answers every request with a 301 to
api.frankfurter.dev/v1, and a redirect response must itself carry
Access-Control-Allow-Origin for a browser to follow it cross-origin. That
301 carries no CORS headers at all, so the request died before the
redirect could be taken.

Measured rather than guessed, with an Origin header set:

  api.frankfurter.app  ->  301, no access-control-* headers
  api.frankfurter.dev  ->  200, access-control-allow-origin: *

The response shape is unchanged, so parseFxResponse needed no edit.

Nothing was wrong at build time, which is why this went unnoticed: a
server-side fetch follows the 301 freely, so the baked rates were always
correct. Only the browser path was broken.

Two changes, because a new default alone fixes only some datasets:

- DEFAULT_FX_API_URL points at .dev/v1, with a test that fails if the
  redirecting host ever comes back.
- A 3 -> 4 migration drops settings.fxApiUrl when it holds exactly the
  dead default, which is the case for anyone who edited that field. It is
  DROPPED rather than rewritten, so the dataset follows whatever the
  current default is and the next move of the service needs no second
  migration for the same user; a URL the owner chose is left strictly
  alone. No stored figure changes, so it cannot alter a number in the
  budget.

Verified by clicking the button in a real browser: "Updated 2 rate(s)",
USD 6.456195 and EUR 7.474959 both stored with source "api" — and the USD
figure differs from the baked constant, so it is a genuine live fetch.

Two migration tests asserted the chain ends at 3; they now assert
SCHEMA_VERSION, since what they mean is "all the way through".

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/store/migrations.test.ts

@@ -166,7 +166,7 @@ describe("v2 -> v3: currencies become data", () => {
 
   test("the three shipped currencies are added, with their decimals", () => {
     const migrated = migrate(v2());
-    expect(migrated.settings.schemaVersion).toBe(3);
+    expect(migrated.settings.schemaVersion).toBe(SCHEMA_VERSION);
     expect(migrated.currencies).toEqual([
       { code: "DKK", digits: 2, symbol: "kr", name: "Danish krone" },
       { code: "USD", digits: 2, symbol: "$", name: "US dollar" },
@@ -196,7 +196,7 @@ describe("v2 -> v3: currencies become data", () => {
     expect(migrated.settings.foldStartMonth).toBe("2026-01");
   });
 
-  test("a v1 dataset migrates all the way through to v3", () => {
+  test("a v1 dataset migrates all the way through to the current version", () => {
     const v1 = {
       settings: { baseCurrency: "DKK", foldStartMonth: "2026-01", schemaVersion: 1 },
       fxRates: [],
@@ -214,7 +214,7 @@ describe("v2 -> v3: currencies become data", () => {
       purchases: [],
     };
     const migrated = migrate(v1);
-    expect(migrated.settings.schemaVersion).toBe(3);
+    expect(migrated.settings.schemaVersion).toBe(SCHEMA_VERSION);
     expect(migrated.posts[0]!.rules).toEqual([
       { from: "2026-01", rule: { kind: "fixed", amount: { amount: 500, currency: "DKK" } } },
     ]);
@@ -233,3 +233,41 @@ describe("v2 -> v3: currencies become data", () => {
     expect(migrated.currencies.map((c) => c.code)).toEqual(["DKK", "JPY"]);
   });
 });
+
+describe("v3 -> v4: the stale rate-service URL is dropped", () => {
+  const v3 = (fxApiUrl?: string) => ({
+    settings: {
+      baseCurrency: "DKK",
+      foldStartMonth: "2026-01",
+      schemaVersion: 3,
+      ...(fxApiUrl === undefined ? {} : { fxApiUrl }),
+    },
+    currencies: [{ code: "DKK", digits: 2 }],
+    fxRates: [],
+    posts: [],
+    months: [],
+    purchases: [],
+  });
+
+  const STALE = "https://api.frankfurter.app/latest?from={base}&to={targets}";
+
+  test("a dataset holding the dead default loses the field entirely", () => {
+    const out = migrate(v3(STALE));
+    expect(out.settings.schemaVersion).toBe(4);
+    // Removed rather than rewritten to the new URL, so the dataset follows
+    // whatever the current default is and a future endpoint move needs no
+    // second migration for the same user.
+    expect("fxApiUrl" in out.settings).toBe(false);
+  });
+
+  test("a URL the owner chose themselves is left alone", () => {
+    const mine = "https://rates.example.com/v2?base={base}&want={targets}";
+    expect(migrate(v3(mine)).settings.fxApiUrl).toBe(mine);
+  });
+
+  test("a dataset that never stored a URL is untouched apart from the version", () => {
+    const out = migrate(v3());
+    expect("fxApiUrl" in out.settings).toBe(false);
+    expect(out.settings.schemaVersion).toBe(4);
+  });
+});