feat(ui): canvas-first redesign with a self-hosted draw.io (#957)

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
- draw.io's own simple UI: its toolbar, menus (auto layout, Mermaid, CSV
  import) and page tabs; the shape library starts closed
- new start screen, session rename and switcher, open .drawio files
- settings split into Models & keys, Appearance, Advanced and About
- Bedrock in the model settings also takes an API key instead of access keys
- phones switch between canvas and chat
- dark mode uses draw.io's own dark mode, with neutral greys around it

With an external draw.io (NEXT_PUBLIC_DRAWIO_BASE_URL) 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/settings.spec.ts

@@ -1,4 +1,10 @@
-import { expect, getIframe, openSettings, test } from "./lib/fixtures"
+import {
+    expect,
+    getIframe,
+    openSettings,
+    openSettingsTab,
+    test,
+} from "./lib/fixtures"
 
 test.describe("Settings", () => {
     test.beforeEach(async ({ page }) => {
@@ -12,14 +18,14 @@ test.describe("Settings", () => {
     })
 
     test("language selection is available", async ({ page }) => {
-        await openSettings(page)
+        await openSettingsTab(page, "appearance")
 
         const dialog = page.locator('[role="dialog"]')
-        await expect(dialog.locator('text="English"')).toBeVisible()
+        await expect(dialog.locator("#language-select")).toHaveText(/English/)
     })
 
     test("max output tokens is editable and persists", async ({ page }) => {
-        await openSettings(page)
+        await openSettingsTab(page, "advanced")
 
         const input = page.locator("#max-output-tokens")
         await expect(input).toBeVisible()
@@ -38,11 +44,14 @@ test.describe("Settings", () => {
         await expect(input).toHaveValue("12000")
     })
 
-    test("draw.io theme toggle exists", async ({ page }) => {
-        await openSettings(page)
+    test("theme can be light, dark or follow the system", async ({ page }) => {
+        await openSettingsTab(page, "appearance")
 
         const dialog = page.locator('[role="dialog"]')
-        const themeText = dialog.locator("text=/sketch|minimal/i")
-        await expect(themeText.first()).toBeVisible()
+        for (const name of ["Light", "Dark", "System"]) {
+            await expect(dialog.getByRole("radio", { name })).toBeVisible()
+        }
+        await dialog.getByRole("radio", { name: "Dark" }).click()
+        await expect(page.locator("html")).toHaveClass(/dark/)
     })
 })