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

@@ -333,14 +333,6 @@ 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>
                 <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" />