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/chat-errors.spec.ts

@@ -32,9 +32,10 @@ test("a rejected key shows a hint, the provider's words and a settings button",
     // First match: the Next.js dev overlay at the end also lists the error
     await expect(page.getByText("Authentication Fails").first()).toBeVisible()
     await page.getByRole("button", { name: "Open model settings" }).click()
-    await expect(
-        page.getByRole("dialog", { name: "AI Model Configuration" }),
-    ).toBeVisible()
+    // Settings opens on its models tab
+    const dialog = page.getByRole("dialog", { name: "Settings" })
+    await expect(dialog).toBeVisible()
+    await expect(dialog.getByText("Providers", { exact: true })).toBeVisible()
 })
 
 test("an error before the stream shows its text, not raw JSON", async ({
@@ -98,7 +99,8 @@ test("a refused server key shows only the quota hint and a settings button", asy
         0,
     )
     await page.getByRole("button", { name: "Open model settings" }).click()
-    await expect(
-        page.getByRole("dialog", { name: "AI Model Configuration" }),
-    ).toBeVisible()
+    // Settings opens on its models tab
+    const dialog = page.getByRole("dialog", { name: "Settings" })
+    await expect(dialog).toBeVisible()
+    await expect(dialog.getByText("Providers", { exact: true })).toBeVisible()
 })