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/settings.spec.ts

@@ -0,0 +1,70 @@
+import { expect, test } from "@playwright/test"
+
+test.describe("Settings", () => {
+    test.beforeEach(async ({ page }) => {
+        await page.goto("/", { waitUntil: "networkidle" })
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+    })
+
+    test("dark mode toggle works", async ({ page }) => {
+        // Open settings
+        const settingsButton = page.locator(
+            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
+        )
+        await settingsButton.click()
+
+        const dialog = page.locator('[role="dialog"]')
+        await expect(dialog).toBeVisible({ timeout: 5000 })
+
+        // Find dark mode toggle
+        const darkModeButton = dialog.locator(
+            'button:has(svg[class*="moon"]), button:has(svg[class*="sun"])',
+        )
+
+        if (await darkModeButton.isVisible()) {
+            // Get initial state
+            const htmlClass = await page.locator("html").getAttribute("class")
+            const wasDark = htmlClass?.includes("dark")
+
+            // Click toggle
+            await darkModeButton.click()
+
+            // Verify state changed
+            const newClass = await page.locator("html").getAttribute("class")
+            const isDark = newClass?.includes("dark")
+
+            expect(isDark).not.toBe(wasDark)
+        }
+    })
+
+    test("language selection is available", async ({ page }) => {
+        // Open settings
+        const settingsButton = page.locator(
+            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
+        )
+        await settingsButton.click()
+
+        const dialog = page.locator('[role="dialog"]')
+        await expect(dialog).toBeVisible({ timeout: 5000 })
+
+        // Should have language selector
+        await expect(dialog.locator('text="English"')).toBeVisible()
+    })
+
+    test("draw.io theme toggle exists", async ({ page }) => {
+        // Open settings
+        const settingsButton = page.locator(
+            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
+        )
+        await settingsButton.click()
+
+        const dialog = page.locator('[role="dialog"]')
+        await expect(dialog).toBeVisible({ timeout: 5000 })
+
+        // Should have draw.io theme option
+        const themeText = dialog.locator("text=/sketch|minimal/i")
+        await expect(themeText.first()).toBeVisible()
+    })
+})