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.

components/workspace/app-toaster.tsx

@@ -0,0 +1,28 @@
+"use client"
+
+import { Toaster } from "sonner"
+import { useSettingsStore } from "@/stores/settings-store"
+
+/** The single toast container, mounted once at the root */
+export function AppToaster() {
+    const isDark = useSettingsStore((s) => s.isDark)
+    return (
+        <Toaster
+            position="bottom-center"
+            // Show stacked toasts in full, so a later one never covers the
+            // button of an earlier one (e.g. "Continue without saving")
+            expand
+            theme={isDark ? "dark" : "light"}
+            offset={{ bottom: 20 }}
+            mobileOffset={{ bottom: 72 }}
+            toastOptions={{
+                duration: 2500,
+                style: { maxWidth: "480px" },
+                classNames: {
+                    toast: "!rounded-xl !border-border !bg-card !text-foreground !shadow-pop",
+                    description: "!text-muted-foreground",
+                },
+            }}
+        />
+    )
+}