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/model-selector.tsx

@@ -26,6 +26,7 @@ import {
     ModelSelectorTrigger,
 } from "@/components/ai-elements/model-selector"
 import { ButtonWithTooltip } from "@/components/button-with-tooltip"
+import { ProviderLogo } from "@/components/provider-logo"
 import { useDictionary } from "@/hooks/use-dictionary"
 import {
     type FlattenedModel,
@@ -66,6 +67,22 @@ function groupModelsByProvider(
     return groups
 }
 
+/**
+ * The model id without the provider or region prefix that the logo already
+ * shows: "nvidia/nemotron-3-ultra" and "global.anthropic.claude-opus-5-5"
+ * become "nemotron-3-ultra" and "claude-opus-5-5". The full id is in the
+ * tooltip and the list.
+ */
+export function shortModelName(id: string): string {
+    const name = id.slice(id.lastIndexOf("/") + 1)
+    return (
+        name.replace(
+            /^(?:(?:global|us|eu|apac|jp|au|ca|us-gov)\.)?[a-z][a-z0-9-]*\.(?=[a-z])/i,
+            "",
+        ) || name
+    )
+}
+
 export function ModelSelector({
     models,
     selectedModelId,
@@ -158,7 +175,7 @@ export function ModelSelector({
     }, [])
 
     return (
-        <div ref={wrapperRef} className="min-w-0 max-w-48">
+        <div ref={wrapperRef} className="min-w-0 max-w-44">
             <ModelSelectorRoot open={open} onOpenChange={setOpen}>
                 <ModelSelectorTrigger asChild>
                     <ButtonWithTooltip
@@ -167,18 +184,26 @@ export function ModelSelector({
                         size="sm"
                         disabled={disabled}
                         className={cn(
-                            "h-8 min-w-0 max-w-full shrink overflow-hidden gap-1.5 px-2 transition-[padding,background-color] duration-150 ease-in-out hover:bg-accent",
+                            "h-8 min-w-0 max-w-full shrink overflow-hidden gap-1.5 px-2 font-normal text-muted-foreground transition-[padding,background-color] duration-150 ease-in-out hover:bg-accent hover:text-foreground",
                             !showLabel && "px-1.5 justify-center",
                         )}
                         // accessibility: expose label to screen readers
                         aria-label={tooltipContent}
+                        data-testid="model-selector"
                     >
-                        <Bot className="h-4 w-4 flex-shrink-0 text-muted-foreground" />
+                        {selectedModel ? (
+                            <ProviderLogo
+                                provider={selectedModel.provider}
+                                className="size-3.5 flex-shrink-0 opacity-80"
+                            />
+                        ) : (
+                            <Bot className="h-4 w-4 flex-shrink-0" />
+                        )}
                         {/* show/hide visible label based on measured width */}
                         {showLabel ? (
                             <span className="min-w-0 truncate text-xs">
                                 {selectedModel
-                                    ? selectedModel.modelId
+                                    ? shortModelName(selectedModel.modelId)
                                     : dict.modelConfig.default}
                             </span>
                         ) : (