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/error-handling.spec.ts

@@ -169,9 +169,14 @@ test.describe("Error Handling", () => {
         await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
         await sendMessage(page, "Draw something")
 
+        // The failed call's row opens to show the error
+        const failed = page
+            .locator('[data-testid="tool-row"][data-tool-state="output-error"]')
+            .first()
+        await failed.getByRole("button").first().click({ timeout: 15000 })
         await expect(
             page.getByText("Invalid input for tool display_diagram").first(),
-        ).toBeVisible({ timeout: 15000 })
+        ).toBeVisible()
         await expect(page.locator('text="Truncated"')).toHaveCount(0)
     })
 })