test: add comprehensive E2E tests for all major features

- Error handling tests (API errors, rate limits, network timeout, truncated XML)
- Multi-turn conversation tests (sequential requests, history preservation)
- File upload tests (upload button, file preview, sending with message)
- Theme switching tests (dark mode toggle, persistence, system preference)
- Language switching tests (EN/JA/ZH, persistence, locale URLs)
- Iframe interaction tests (draw.io loading, toolbar, diagram rendering)
- Copy/paste tests (chat input, XML input, special characters)
- History restore tests (new chat, persistence, browser navigation)

tests/e2e/error-handling.spec.ts

@@ -0,0 +1,144 @@
+import { expect, test } from "@playwright/test"
+
+test.describe("Error Handling", () => {
+    test("displays error message when API returns 500", async ({ page }) => {
+        await page.route("**/api/chat", async (route) => {
+            await route.fulfill({
+                status: 500,
+                contentType: "application/json",
+                body: JSON.stringify({ error: "Internal server error" }),
+            })
+        })
+
+        await page.goto("/", { waitUntil: "networkidle" })
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+
+        const chatInput = page.locator('textarea[aria-label="Chat input"]')
+        await expect(chatInput).toBeVisible({ timeout: 10000 })
+
+        await chatInput.fill("Draw a cat")
+        await chatInput.press("ControlOrMeta+Enter")
+
+        // After error, user should be able to type again (input still functional)
+        await page.waitForTimeout(2000) // Wait for error response
+        await chatInput.fill("Retry message")
+        await expect(chatInput).toHaveValue("Retry message")
+    })
+
+    test("displays error message when API returns 429 rate limit", async ({
+        page,
+    }) => {
+        await page.route("**/api/chat", async (route) => {
+            await route.fulfill({
+                status: 429,
+                contentType: "application/json",
+                body: JSON.stringify({ error: "Rate limit exceeded" }),
+            })
+        })
+
+        await page.goto("/", { waitUntil: "networkidle" })
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+
+        const chatInput = page.locator('textarea[aria-label="Chat input"]')
+        await expect(chatInput).toBeVisible({ timeout: 10000 })
+
+        await chatInput.fill("Draw a cat")
+        await chatInput.press("ControlOrMeta+Enter")
+
+        // After rate limit error, user should be able to type again
+        await page.waitForTimeout(2000) // Wait for error response
+        await chatInput.fill("Retry after rate limit")
+        await expect(chatInput).toHaveValue("Retry after rate limit")
+    })
+
+    test("handles network timeout gracefully", async ({ page }) => {
+        await page.route("**/api/chat", async (route) => {
+            // Simulate timeout by not responding for a short time then aborting
+            await new Promise((resolve) => setTimeout(resolve, 2000))
+            await route.abort("timedout")
+        })
+
+        await page.goto("/", { waitUntil: "networkidle" })
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+
+        const chatInput = page.locator('textarea[aria-label="Chat input"]')
+        await expect(chatInput).toBeVisible({ timeout: 10000 })
+
+        await chatInput.fill("Draw a cat")
+        await chatInput.press("ControlOrMeta+Enter")
+
+        // Wait for timeout error to occur
+        await page.waitForTimeout(3000)
+
+        // After timeout, user should be able to type again
+        await chatInput.fill("Try again after timeout")
+        await expect(chatInput).toHaveValue("Try again after timeout")
+    })
+
+    test("shows truncated badge for incomplete XML", async ({ page }) => {
+        const toolCallId = `call_${Date.now()}`
+        const textId = `text_${Date.now()}`
+        const messageId = `msg_${Date.now()}`
+
+        // Truncated XML (missing closing tags)
+        const truncatedXml = `<mxCell id="node1" value="Start" style="rounded=1;" vertex="1" parent="1">
+  <mxGeometry x="100" y="100" width="100" height="40"`
+
+        const events = [
+            { type: "start", messageId },
+            { type: "text-start", id: textId },
+            { type: "text-delta", id: textId, delta: "Creating diagram..." },
+            { type: "text-end", id: textId },
+            {
+                type: "tool-input-start",
+                toolCallId,
+                toolName: "display_diagram",
+            },
+            {
+                type: "tool-input-available",
+                toolCallId,
+                toolName: "display_diagram",
+                input: { xml: truncatedXml },
+            },
+            {
+                type: "tool-output-error",
+                toolCallId,
+                error: "XML validation failed",
+            },
+            { type: "finish" },
+        ]
+
+        await page.route("**/api/chat", async (route) => {
+            await route.fulfill({
+                status: 200,
+                contentType: "text/event-stream",
+                body:
+                    events
+                        .map((e) => `data: ${JSON.stringify(e)}\n\n`)
+                        .join("") + "data: [DONE]\n\n",
+            })
+        })
+
+        await page.goto("/", { waitUntil: "networkidle" })
+        await page
+            .locator("iframe")
+            .waitFor({ state: "visible", timeout: 30000 })
+
+        const chatInput = page.locator('textarea[aria-label="Chat input"]')
+        await expect(chatInput).toBeVisible({ timeout: 10000 })
+
+        await chatInput.fill("Draw something")
+        await chatInput.press("ControlOrMeta+Enter")
+
+        // Should show truncated badge
+        await expect(page.locator('text="Truncated"')).toBeVisible({
+            timeout: 15000,
+        })
+    })
+})