test: add more E2E tests for UI components

- Chat panel tests (interactive elements, iframe)
- Settings tests (dark mode, language, draw.io theme)
- Save dialog tests (buttons exist)
- History dialog tests
- Model config tests
- Keyboard interaction tests
- Upload area tests

Total: 15 E2E tests, all passing

tests/e2e/chat.spec.ts

@@ -0,0 +1,26 @@
+import { expect, test } from "@playwright/test"
+
+test.describe("Chat Panel", () => {
+    test.beforeEach(async ({ page }) => {
+        await page.goto("/", { waitUntil: "networkidle" })
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+    })
+
+    test("page has interactive elements", async ({ page }) => {
+        // Verify buttons exist (settings, etc.)
+        const buttons = page.locator("button")
+        const count = await buttons.count()
+        expect(count).toBeGreaterThan(0)
+    })
+
+    test("draw.io iframe is interactive", async ({ page }) => {
+        const iframe = page.locator("iframe")
+        await expect(iframe).toBeVisible()
+
+        // Iframe should have loaded draw.io
+        const src = await iframe.getAttribute("src")
+        expect(src).toBeTruthy()
+    })
+})