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/model-test.spec.ts

@@ -48,7 +48,7 @@ test("the Test button checks all models at once and shows each result", async ({
     await page.goto("/", { waitUntil: "networkidle" })
     await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
 
-    await page.locator("button:has(svg.lucide-bot)").first().click()
+    await page.getByTestId("model-selector").click()
     await page.getByText("Configure Models...").click()
     const dialog = page.locator('[role="dialog"]')
     await dialog.getByText("GLM (Zhipu)").first().click()
@@ -75,7 +75,7 @@ test("the key link and the base URL cleanup", async ({ page }) => {
     }, CONFIG)
     await page.goto("/", { waitUntil: "networkidle" })
     await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
-    await page.locator("button:has(svg.lucide-bot)").first().click()
+    await page.getByTestId("model-selector").click()
     await page.getByText("Configure Models...").click()
     const dialog = page.locator('[role="dialog"]')
     await dialog.getByText("GLM (Zhipu)").first().click()