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.

components/canvas/selection-ask.tsx

@@ -0,0 +1,71 @@
+import { Sparkles } from "lucide-react"
+import { useDictionary } from "@/hooks/use-dictionary"
+import { modKey } from "@/lib/platform"
+import { useCanvasStore } from "@/stores/canvas-store"
+import { useUiStore } from "@/stores/ui-store"
+
+const BUTTON_HEIGHT = 32
+const GAP = 10
+
+/**
+ * "Ask AI to change this" next to the selected shapes. Clicking it moves
+ * focus to the chat input, where the selection already shows as a chip.
+ */
+export function SelectionAsk({
+    width,
+    height,
+    hidden,
+}: {
+    /** Size of the canvas area, for keeping the button inside it */
+    width: number
+    height: number
+    hidden: boolean
+}) {
+    const dict = useDictionary()
+    const rect = useCanvasStore((s) => s.selectionRect)
+    const selection = useCanvasStore((s) => s.selection)
+    const isFreehand = useCanvasStore((s) => s.isFreehand)
+    const popupOpen = useCanvasStore((s) => s.isDrawioPopupOpen)
+    const focusComposer = useUiStore((s) => s.focusComposer)
+
+    if (hidden || popupOpen || !rect || selection.length === 0 || isFreehand) {
+        return null
+    }
+    // Selection scrolled out of view
+    if (
+        rect.x + rect.width < 0 ||
+        rect.y + rect.height < 0 ||
+        rect.x > width ||
+        rect.y > height
+    ) {
+        return null
+    }
+
+    // Above the selection's top-right corner, kept inside the canvas
+    const approxWidth = 168
+    const left = Math.min(
+        Math.max(8, rect.x + rect.width - approxWidth / 2),
+        width - approxWidth - 8,
+    )
+    const above = rect.y - BUTTON_HEIGHT - GAP
+    const top =
+        above > 64
+            ? above
+            : Math.min(rect.y + rect.height + GAP, height - BUTTON_HEIGHT - 72)
+
+    return (
+        <button
+            type="button"
+            onClick={focusComposer}
+            className="pointer-events-auto absolute z-10 flex h-8 items-center gap-1.5 rounded-xl bg-card pr-2.5 pl-2 text-[13px] font-medium text-foreground shadow-pop transition-transform animate-fade-in hover:-translate-y-px"
+            style={{ left, top }}
+            data-testid="selection-ask"
+        >
+            <Sparkles className="size-4 text-marker-ink" />
+            {dict.workspace.askAi}
+            <kbd className="ml-1 font-sans text-[11px] text-muted-foreground">
+                {modKey}/
+            </kbd>
+        </button>
+    )
+}