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/relative-time.ts

@@ -0,0 +1,20 @@
+/** "3 min ago", "yesterday", or a short date, in the UI language */
+export function formatRelativeTime(
+    timestamp: number,
+    locale: string,
+    justNow: string,
+): string {
+    const diffMs = Date.now() - timestamp
+    const minutes = Math.floor(diffMs / 60_000)
+    if (minutes < 1) return justNow
+    const rtf = new Intl.RelativeTimeFormat(locale, { numeric: "auto" })
+    if (minutes < 60) return rtf.format(-minutes, "minute")
+    const hours = Math.floor(minutes / 60)
+    if (hours < 24) return rtf.format(-hours, "hour")
+    const days = Math.floor(hours / 24)
+    if (days < 7) return rtf.format(-days, "day")
+    return new Date(timestamp).toLocaleDateString(locale, {
+        month: "short",
+        day: "numeric",
+    })
+}