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.

hooks/use-panel-width.ts

@@ -0,0 +1,22 @@
+import { useEffect, useState } from "react"
+import { PANEL_MIN_WIDTH, useSettingsStore } from "@/stores/settings-store"
+
+// Room the canvas keeps next to the chat panel in a small window
+const MIN_CANVAS_WIDTH = 400
+
+/** The chat panel's width, narrower when the window has no room for it */
+export function usePanelWidth(): number {
+    const width = useSettingsStore((s) => s.panelWidth)
+    // Set after mounting, so the server render and the first one agree
+    const [windowWidth, setWindowWidth] = useState(Number.POSITIVE_INFINITY)
+    useEffect(() => {
+        const update = () => setWindowWidth(window.innerWidth)
+        update()
+        window.addEventListener("resize", update)
+        return () => window.removeEventListener("resize", update)
+    }, [])
+    return Math.max(
+        PANEL_MIN_WIDTH,
+        Math.min(width, windowWidth - MIN_CANVAS_WIDTH),
+    )
+}