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/chat/TemplatePanel.tsx

@@ -36,6 +36,7 @@ import {
     updateTemplate,
     validateImportData,
 } from "@/lib/template-storage"
+import { useUiStore } from "@/stores/ui-store"
 import { TemplateCreateDialog } from "./TemplateCreateDialog"
 import { TemplateEditDialog } from "./TemplateEditDialog"
 
@@ -43,6 +44,8 @@ interface TemplatePanelProps {
     setInput: (input: string) => void
     onSendTemplate?: (template: Template) => void
     currentInput?: string
+    /** Skip the title block (the surrounding dialog already has one) */
+    hideHeader?: boolean
 }
 
 function formatLastUsed(timestamp: number, neverUsedText: string): string {
@@ -77,8 +80,10 @@ export function TemplatePanel({
     setInput,
     onSendTemplate,
     currentInput = "",
+    hideHeader = false,
 }: TemplatePanelProps) {
     const dict = useDictionary()
+    const templatesVersion = useUiStore((s) => s.templatesVersion)
     const [templates, setTemplates] = useState<Template[]>([])
     const [loading, setLoading] = useState(true)
     const [createDialogOpen, setCreateDialogOpen] = useState(false)
@@ -108,13 +113,10 @@ export function TemplatePanel({
         ? searchTemplates(templates, searchQuery)
         : templates
 
+    // Reload when a template is saved elsewhere, e.g. from the chat input
     useEffect(() => {
         loadTemplates()
-        // Reload when a template is saved elsewhere, e.g. from the chat input
-        window.addEventListener("templatesChanged", loadTemplates)
-        return () =>
-            window.removeEventListener("templatesChanged", loadTemplates)
-    }, [loadTemplates])
+    }, [loadTemplates, templatesVersion])
 
     const handleCreateSuccess = () => {
         loadTemplates()
@@ -333,14 +335,16 @@ export function TemplatePanel({
     if (!loading && templates.length === 0) {
         return (
             <div className="py-6 px-2 animate-fade-in">
-                <div className="text-center mb-6">
-                    <h2 className="text-lg font-semibold text-foreground mb-2">
-                        {dict.templates.title}
-                    </h2>
-                    <p className="text-sm text-muted-foreground max-w-xs mx-auto">
-                        {dict.templates.subtitle}
-                    </p>
-                </div>
+                {!hideHeader && (
+                    <div className="text-center mb-6">
+                        <h2 className="text-lg font-semibold text-foreground mb-2">
+                            {dict.templates.title}
+                        </h2>
+                        <p className="text-sm text-muted-foreground max-w-xs mx-auto">
+                            {dict.templates.subtitle}
+                        </p>
+                    </div>
+                )}
                 <div className="flex flex-col items-center justify-center py-8 px-4">
                     <div className="w-16 h-16 rounded-2xl bg-primary/10 flex items-center justify-center mb-4">
                         <FileText className="w-8 h-8 text-primary/60" />