feat(ui): canvas-first redesign with a self-hosted draw.io

The canvas now fills the window and the chat floats on its right. draw.io is
served from our own origin (public/drawio, downloaded at build time and
checked against the release's SHA-256), so the app can drive the editor
directly:

- every AI change becomes a version card with a thumbnail; compare, restore
  or undo it, and Ctrl+Z on the canvas takes an AI change back in one step
- shapes the AI just changed are highlighted on the canvas
- selected shapes appear as a chip and are sent to the model
- the app's own canvas toolbar, shape library and page tabs replace
  draw.io's chrome
- new start screen, session rename and switcher, open .drawio files
- settings split into Models & keys, Appearance, Advanced and About
- phones switch between canvas and chat
- dark mode keeps the diagram on a white sheet and no longer flashes

With an external draw.io (NEXT_PUBLIC_DRAWIO_BASE_URL) the app falls back to
draw.io's own toolbar and the features above that need same origin are off.

The chat logic moved from the 1500-line chat panel into a chat engine plus
small UI components; zustand holds canvas, versions, settings and UI state.

tests/e2e/language.spec.ts

@@ -2,7 +2,8 @@ import {
     expect,
     getChatInput,
     getIframe,
-    openSettings,
+    getSendButton,
+    openSettingsTab,
     sleep,
     test,
 } from "./lib/fixtures"
@@ -15,7 +16,7 @@ test.describe("Language Switching", () => {
         const chatInput = getChatInput(page)
         await expect(chatInput).toBeVisible({ timeout: 10000 })
 
-        await expect(page.locator('button:has-text("Send")')).toBeVisible()
+        await expect(getSendButton(page, "Send")).toBeVisible()
     })
 
     test("can switch to Japanese", async ({ page }) => {
@@ -23,14 +24,16 @@ test.describe("Language Switching", () => {
         await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
 
         await test.step("open settings and select Japanese", async () => {
-            await openSettings(page)
-            const languageSelector = page.locator('button:has-text("English")')
-            await languageSelector.first().click()
-            await page.locator('text="日本語"').click()
+            await openSettingsTab(page, "appearance")
+            await page.locator("#language-select").click()
+            await page
+                .getByRole("option", { name: "日本語", exact: true })
+                .click()
+            await page.keyboard.press("Escape")
         })
 
         await test.step("verify UI is in Japanese", async () => {
-            await expect(page.locator('button:has-text("送信")')).toBeVisible({
+            await expect(getSendButton(page, "送信")).toBeVisible({
                 timeout: 5000,
             })
         })
@@ -41,14 +44,16 @@ test.describe("Language Switching", () => {
         await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
 
         await test.step("open settings and select Chinese", async () => {
-            await openSettings(page)
-            const languageSelector = page.locator('button:has-text("English")')
-            await languageSelector.first().click()
-            await page.locator('text="中文"').click()
+            await openSettingsTab(page, "appearance")
+            await page.locator("#language-select").click()
+            await page
+                .getByRole("option", { name: "中文", exact: true })
+                .click()
+            await page.keyboard.press("Escape")
         })
 
         await test.step("verify UI is in Chinese", async () => {
-            await expect(page.locator('button:has-text("发送")')).toBeVisible({
+            await expect(getSendButton(page, "发送")).toBeVisible({
                 timeout: 5000,
             })
         })
@@ -59,16 +64,18 @@ test.describe("Language Switching", () => {
         await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
 
         await test.step("switch to Japanese", async () => {
-            await openSettings(page)
-            const languageSelector = page.locator('button:has-text("English")')
-            await languageSelector.first().click()
-            await page.locator('text="日本語"').click()
+            await openSettingsTab(page, "appearance")
+            await page.locator("#language-select").click()
+            await page
+                .getByRole("option", { name: "日本語", exact: true })
+                .click()
+            await page.keyboard.press("Escape")
             await page.keyboard.press("Escape")
             await sleep(500)
         })
 
         await test.step("verify Japanese before reload", async () => {
-            await expect(page.locator('button:has-text("送信")')).toBeVisible({
+            await expect(getSendButton(page, "送信")).toBeVisible({
                 timeout: 10000,
             })
         })
@@ -78,7 +85,7 @@ test.describe("Language Switching", () => {
             await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
             // Wait for hydration and localStorage to be read
             await sleep(1000)
-            await expect(page.locator('button:has-text("送信")')).toBeVisible({
+            await expect(getSendButton(page, "送信")).toBeVisible({
                 timeout: 10000,
             })
         })
@@ -88,7 +95,7 @@ test.describe("Language Switching", () => {
         await page.goto("/ja", { waitUntil: "networkidle" })
         await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
 
-        await expect(page.locator('button:has-text("送信")')).toBeVisible({
+        await expect(getSendButton(page, "送信")).toBeVisible({
             timeout: 10000,
         })
     })
@@ -97,7 +104,7 @@ test.describe("Language Switching", () => {
         await page.goto("/zh", { waitUntil: "networkidle" })
         await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
 
-        await expect(page.locator('button:has-text("发送")')).toBeVisible({
+        await expect(getSendButton(page, "发送")).toBeVisible({
             timeout: 10000,
         })
     })