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.

lib/version-text.ts

@@ -0,0 +1,47 @@
+import type { ChangeSummary } from "@/lib/diagram-diff"
+import { formatMessage } from "@/lib/i18n/utils"
+
+type VersionDict = Record<string, string>
+
+const PARTS: [keyof ChangeSummary, string][] = [
+    ["shapesAdded", "shapesAdded"],
+    ["edgesAdded", "edgesAdded"],
+    ["shapesChanged", "shapesChanged"],
+    ["edgesChanged", "edgesChanged"],
+    ["shapesRemoved", "shapesRemoved"],
+    ["edgesRemoved", "edgesRemoved"],
+]
+
+/**
+ * "Added 2 shapes, changed 1 connector". Each dictionary entry has a
+ * singular (...One) and plural (...Other) form; languages without plurals
+ * use the same text for both.
+ */
+export function describeChanges(
+    summary: ChangeSummary,
+    t: VersionDict,
+): string {
+    const pieces = PARTS.filter(([field]) => summary[field] > 0).map(
+        ([field, key]) => {
+            const count = summary[field]
+            const template = count === 1 ? t[`${key}One`] : t[`${key}Other`]
+            return formatMessage(template ?? "", { count })
+        },
+    )
+    if (pieces.length === 0) return t.noChanges ?? ""
+    const text = pieces.join(t.listSeparator ?? ", ")
+    // Sentence case for scripts with letter case
+    return text.charAt(0).toUpperCase() + text.slice(1)
+}
+
+/** "3 shapes, 1 connector": what a diagram drawn from scratch holds */
+export function describeTotals(summary: ChangeSummary, t: VersionDict): string {
+    const count = (n: number, key: string) =>
+        formatMessage((n === 1 ? t[`${key}One`] : t[`${key}Other`]) ?? "", {
+            count: n,
+        })
+    return formatMessage(t.totals ?? "", {
+        shapes: count(summary.shapesAdded, "shapesTotal"),
+        edges: count(summary.edgesAdded, "edgesTotal"),
+    })
+}