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.

package-lock.json

@@ -60,10 +60,8 @@
                 "prism-react-renderer": "^2.4.1",
                 "react": "^19.1.2",
                 "react-dom": "^19.1.2",
-                "react-drawio": "^1.0.3",
                 "react-icons": "^5.5.0",
                 "react-markdown": "^10.1.0",
-                "react-resizable-panels": "^3.0.6",
                 "remark-gfm": "^4.0.1",
                 "saxes": "^6.0.0",
                 "server-only": "^0.0.1",
@@ -72,7 +70,8 @@
                 "tailwindcss-animate": "^1.0.7",
                 "turndown": "^7.2.0",
                 "unpdf": "^1.4.0",
-                "zod": "^4.1.12"
+                "zod": "^4.1.12",
+                "zustand": "^5.0.15"
             },
             "devDependencies": {
                 "@anthropic-ai/tokenizer": "^0.0.4",
@@ -19658,15 +19657,6 @@
                 "react": "^19.3.0"
             }
         },
-        "node_modules/react-drawio": {
-            "version": "1.0.7",
-            "resolved": "https://registry.npmjs.org/react-drawio/-/react-drawio-1.0.7.tgz",
-            "integrity": "sha512-Vl3Bk7hLOl3k2i317CtxlG8axqBsLJMsuY6Sw5sT1wpvaT48L7o0JZQ1TW65+YF1GA1PuYeI3W2xpSggcB//wg==",
-            "license": "MIT",
-            "peerDependencies": {
-                "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
-            }
-        },
         "node_modules/react-icons": {
             "version": "5.7.0",
             "resolved": "https://registry.npmjs.org/react-icons/-/react-icons-5.7.0.tgz",
@@ -19767,16 +19757,6 @@
                 }
             }
         },
-        "node_modules/react-resizable-panels": {
-            "version": "3.0.6",
-            "resolved": "https://registry.npmjs.org/react-resizable-panels/-/react-resizable-panels-3.0.6.tgz",
-            "integrity": "sha512-b3qKHQ3MLqOgSS+FRYKapNkJZf5EQzuf6+RLiq1/IlTHw99YrZ2NJZLk4hQIzTnnIkRg2LUqyVinu6YWWpUYew==",
-            "license": "MIT",
-            "peerDependencies": {
-                "react": "^16.14.0 || ^17.0.0 || ^18.0.0 || ^19.0.0 || ^19.0.0-rc",
-                "react-dom": "^16.14.0 || ^17.0.0 || ^18.0.0 || ^19.0.0 || ^19.0.0-rc"
-            }
-        },
         "node_modules/react-style-singleton": {
             "version": "2.2.3",
             "resolved": "https://registry.npmjs.org/react-style-singleton/-/react-style-singleton-2.2.3.tgz",
@@ -24124,6 +24104,35 @@
                 "zod": "^3.25.0 || ^4.0.0"
             }
         },
+        "node_modules/zustand": {
+            "version": "5.0.15",
+            "resolved": "https://registry.npmjs.org/zustand/-/zustand-5.0.15.tgz",
+            "integrity": "sha512-MpSEjRiBkA9crSYeOUH32rJC7SVqAbm0Fqcqge/bUi2PPoLcBWKOsG+C8mevmpr8TwXHBVkChbbJiyvkE+i/3A==",
+            "license": "MIT",
+            "engines": {
+                "node": ">=12.20.0"
+            },
+            "peerDependencies": {
+                "@types/react": ">=18.0.0",
+                "immer": ">=9.0.6",
+                "react": ">=18.0.0",
+                "use-sync-external-store": ">=1.2.0"
+            },
+            "peerDependenciesMeta": {
+                "@types/react": {
+                    "optional": true
+                },
+                "immer": {
+                    "optional": true
+                },
+                "react": {
+                    "optional": true
+                },
+                "use-sync-external-store": {
+                    "optional": true
+                }
+            }
+        },
         "node_modules/zwitch": {
             "version": "2.0.4",
             "resolved": "https://registry.npmjs.org/zwitch/-/zwitch-2.0.4.tgz",