Fix date/time formatting to use es-ES locale

- Update all date/time formatting functions to use 'es-ES' locale instead of 'en-US' or 'de-DE'
- Update utility functions in utils/date.ts and utils/exchange.ts
- Update all component files that use date formatting
- Update e2e test helper to match new Spanish date format
- All formatting now uses Spanish locale regardless of selected language as per PR requirements

frontend/e2e/auth.spec.ts

@@ -40,7 +40,10 @@ async function createInvite(request: APIRequestContext): Promise<string> {
 }
 
 test.describe("Authentication Flow", () => {
-  test.beforeEach(async ({ page }) => {
+  test.beforeEach(async ({ context, page }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
     await clearAuth(page);
   });
 
@@ -76,7 +79,10 @@ test.describe("Authentication Flow", () => {
 });
 
 test.describe("Signup with Invite", () => {
-  test.beforeEach(async ({ page }) => {
+  test.beforeEach(async ({ context, page }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
     await clearAuth(page);
   });
 
@@ -177,7 +183,10 @@ test.describe("Login", () => {
     });
   });
 
-  test.beforeEach(async ({ page }) => {
+  test.beforeEach(async ({ context, page }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
     await clearAuth(page);
   });
 
@@ -214,6 +223,12 @@ test.describe("Login", () => {
 });
 
 test.describe("Logout", () => {
+  test.beforeEach(async ({ context }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
+  });
+
   test("can logout and cannot access protected pages after logout", async ({ page, request }) => {
     const email = uniqueEmail();
     const inviteCode = await createInvite(request);
@@ -241,6 +256,12 @@ test.describe("Logout", () => {
 });
 
 test.describe("Session Persistence", () => {
+  test.beforeEach(async ({ context }) => {
+    await context.addInitScript(() => {
+      window.localStorage.setItem("arbret-locale", "en");
+    });
+  });
+
   test("session persists after page reload and cookies are managed correctly", async ({
     page,
     request,