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.

docker-compose.yml

@@ -1,14 +1,11 @@
 services:
-  drawio:
-    image: jgraph/drawio:latest
-    ports: ["8080:8080"]
   next-ai-draw-io:
     build:
       context: .
-      args:
-        - NEXT_PUBLIC_DRAWIO_BASE_URL=http://localhost:8080
-        # Uncomment below for subdirectory deployment
-        # - NEXT_PUBLIC_BASE_PATH=/nextaidrawio
+      # draw.io is bundled into the image (public/drawio), no separate container needed
+      # Uncomment below for subdirectory deployment
+      # args:
+      #   - NEXT_PUBLIC_BASE_PATH=/nextaidrawio
     ports: ["3000:3000"]
     env_file: .env
     volumes:
@@ -17,4 +14,3 @@ services:
     # environment:
     #   # For subdirectory deployment, uncomment and set your path:
     #   NEXT_PUBLIC_BASE_PATH: /nextaidrawio
-    depends_on: [drawio]