test: add Vitest and Playwright testing infrastructure

- Add Vitest for unit tests (39 tests)
  - cached-responses.test.ts
  - ai-providers.test.ts
  - chat-helpers.test.ts
  - utils.test.ts
- Add Playwright for E2E tests (3 smoke tests)
  - Homepage load
  - Japanese locale
  - Settings dialog
- Add CI workflow (.github/workflows/test.yml)
- Add vitest.config.mts and playwright.config.ts
- Update .gitignore for test artifacts

tests/e2e/smoke.spec.ts

@@ -0,0 +1,51 @@
+import { expect, test } from "@playwright/test"
+
+test.describe("Smoke Tests", () => {
+    test("homepage loads without errors", async ({ page }) => {
+        const errors: string[] = []
+        page.on("pageerror", (err) => errors.push(err.message))
+
+        await page.goto("/", { waitUntil: "networkidle" })
+        await expect(page).toHaveTitle(/Draw\.io/i, { timeout: 10000 })
+
+        // Wait for draw.io iframe to be present
+        const iframe = page.locator("iframe")
+        await expect(iframe).toBeVisible({ timeout: 30000 })
+
+        expect(errors).toEqual([])
+    })
+
+    test("Japanese locale page loads", async ({ page }) => {
+        const errors: string[] = []
+        page.on("pageerror", (err) => errors.push(err.message))
+
+        await page.goto("/ja", { waitUntil: "networkidle" })
+        await expect(page).toHaveTitle(/Draw\.io/i, { timeout: 10000 })
+
+        const iframe = page.locator("iframe")
+        await expect(iframe).toBeVisible({ timeout: 30000 })
+
+        expect(errors).toEqual([])
+    })
+
+    test("settings dialog opens", async ({ page }) => {
+        await page.goto("/")
+
+        // Wait for page to load
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+
+        // Click settings button (gear icon)
+        const settingsButton = page.locator(
+            'button[aria-label*="settings"], button:has(svg[class*="lucide-settings"])',
+        )
+        if (await settingsButton.isVisible()) {
+            await settingsButton.click()
+            // Check if settings dialog appears
+            await expect(page.locator('[role="dialog"]')).toBeVisible({
+                timeout: 5000,
+            })
+        }
+    })
+})