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/ui/resizable.tsx

@@ -1,56 +0,0 @@
-"use client"
-
-import * as React from "react"
-import { GripVerticalIcon } from "lucide-react"
-import * as ResizablePrimitive from "react-resizable-panels"
-
-import { cn } from "@/lib/utils"
-
-function ResizablePanelGroup({
-  className,
-  ...props
-}: React.ComponentProps<typeof ResizablePrimitive.PanelGroup>) {
-  return (
-    <ResizablePrimitive.PanelGroup
-      data-slot="resizable-panel-group"
-      className={cn(
-        "flex h-full w-full data-[panel-group-direction=vertical]:flex-col",
-        className
-      )}
-      {...props}
-    />
-  )
-}
-
-function ResizablePanel({
-  ...props
-}: React.ComponentProps<typeof ResizablePrimitive.Panel>) {
-  return <ResizablePrimitive.Panel data-slot="resizable-panel" {...props} />
-}
-
-function ResizableHandle({
-  withHandle,
-  className,
-  ...props
-}: React.ComponentProps<typeof ResizablePrimitive.PanelResizeHandle> & {
-  withHandle?: boolean
-}) {
-  return (
-    <ResizablePrimitive.PanelResizeHandle
-      data-slot="resizable-handle"
-      className={cn(
-        "bg-border focus-visible:ring-ring relative flex w-px items-center justify-center after:absolute after:inset-y-0 after:left-1/2 after:w-1 after:-translate-x-1/2 focus-visible:ring-1 focus-visible:ring-offset-1 focus-visible:outline-hidden data-[panel-group-direction=vertical]:h-px data-[panel-group-direction=vertical]:w-full data-[panel-group-direction=vertical]:after:left-0 data-[panel-group-direction=vertical]:after:h-1 data-[panel-group-direction=vertical]:after:w-full data-[panel-group-direction=vertical]:after:translate-x-0 data-[panel-group-direction=vertical]:after:-translate-y-1/2 [&[data-panel-group-direction=vertical]>div]:rotate-90",
-        className
-      )}
-      {...props}
-    >
-      {withHandle && (
-        <div className="bg-border z-10 flex h-4 w-3 items-center justify-center rounded-xs border">
-          <GripVerticalIcon className="size-2.5" />
-        </div>
-      )}
-    </ResizablePrimitive.PanelResizeHandle>
-  )
-}
-
-export { ResizablePanelGroup, ResizablePanel, ResizableHandle }