feat(ui): canvas-first redesign with a self-hosted draw.io (#957)

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
- draw.io's own simple UI: its toolbar, menus (auto layout, Mermaid, CSV
  import) and page tabs; the shape library starts closed
- new start screen, session rename and switcher, open .drawio files
- settings split into Models & keys, Appearance, Advanced and About
- Bedrock in the model settings also takes an API key instead of access keys
- phones switch between canvas and chat
- dark mode uses draw.io's own dark mode, with neutral greys around it

With an external draw.io (NEXT_PUBLIC_DRAWIO_BASE_URL) 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.

tests/unit/tool-activity.test.tsx

@@ -0,0 +1,50 @@
+import { cleanup, render } from "@testing-library/react"
+import { afterEach, describe, expect, it } from "vitest"
+import { ToolInputDetails } from "@/components/chat/tool-activity"
+
+afterEach(cleanup)
+
+describe("ToolInputDetails", () => {
+    it("shows streamed operations without crashing on broken entries", () => {
+        // A partly streamed or invalid edit_diagram input
+        const operations = [
+            null,
+            { operation: {} },
+            { operation: "add", cell_id: {} },
+            // JSON can hold an object that does not turn into text
+            JSON.parse('{"operation":"add","cell_id":{"toString":null}}'),
+            { operation: "add", cell_id: "2", new_xml: {} },
+            { operation: "update", cell_id: "3", new_xml: '<mxCell id="3"/>' },
+        ]
+        const { container } = render(
+            <ToolInputDetails
+                part={
+                    {
+                        type: "tool-edit_diagram",
+                        toolCallId: "t1",
+                        state: "input-streaming",
+                        input: { operations },
+                    } as any
+                }
+            />,
+        )
+        expect(container.textContent).toContain("update3")
+        expect(container.textContent).toContain('<mxCell id="3"/>')
+    })
+
+    it("ignores an xml input that is not text", () => {
+        const { container } = render(
+            <ToolInputDetails
+                part={
+                    {
+                        type: "tool-display_diagram",
+                        toolCallId: "t2",
+                        state: "input-streaming",
+                        input: { xml: { broken: true } },
+                    } as any
+                }
+            />,
+        )
+        expect(container.textContent).toContain("broken")
+    })
+})