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/utils.ts

@@ -341,13 +341,9 @@ export function extractDiagramXML(xml_svg_string: string): string {
             throw new Error("SVG element does not have a 'content' attribute.")
         }
 
-        // 3. Decode HTML entities (using a minimal function)
-        function decodeHtmlEntities(str: string) {
-            const textarea = document.createElement("textarea") // Use built-in element
-            textarea.innerHTML = str
-            return textarea.value
-        }
-        const xmlContent = decodeHtmlEntities(encodedContent)
+        // 3. getAttribute already decoded the entities: decoding again
+        // would break values that hold them (a page named "R & D")
+        const xmlContent = encodedContent
 
         // 4. Parse the XML content
         const xmlDoc = parser.parseFromString(xmlContent, "text/xml")