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/chat/message-text.ts

@@ -0,0 +1,64 @@
+import type { UIMessage } from "ai"
+
+/** A run of plain text, or an attached file / URL whose text was inlined */
+export interface TextSection {
+    type: "text" | "file" | "url"
+    content: string
+    filename?: string
+    charCount?: number
+    fileType?: "pdf" | "text" | "url"
+}
+
+// Matches the [PDF: ...], [File: ...] and [URL: ...] sections appended to the user's text
+export const APPENDED_FILE_SECTIONS_PATTERN =
+    /\n\n\[(PDF|File|URL):\s*[^\]]+\]\n[\s\S]*$/
+
+export function splitTextIntoFileSections(text: string): TextSection[] {
+    const sections: TextSection[] = []
+    const filePattern =
+        /\[(PDF|File|URL):\s*([^\]]+)\]\n([\s\S]*?)(?=\n\n\[(PDF|File|URL):|$)/g
+    let lastIndex = 0
+    let match: RegExpExecArray | null
+
+    while ((match = filePattern.exec(text)) !== null) {
+        const beforeText = text.slice(lastIndex, match.index).trim()
+        if (beforeText) sections.push({ type: "text", content: beforeText })
+
+        const sectionType = match[1].toLowerCase()
+        const fileType =
+            sectionType === "pdf"
+                ? "pdf"
+                : sectionType === "url"
+                  ? "url"
+                  : "text"
+        const content = match[3].trim()
+        sections.push({
+            type: sectionType === "url" ? "url" : "file",
+            content,
+            filename: match[2].trim(),
+            charCount: content.length,
+            fileType,
+        })
+        lastIndex = match.index + match[0].length
+    }
+
+    const remainingText = text.slice(lastIndex).trim()
+    if (remainingText) sections.push({ type: "text", content: remainingText })
+    if (sections.length === 0) sections.push({ type: "text", content: text })
+    return sections
+}
+
+export function getMessageTextContent(message: UIMessage): string {
+    if (!message.parts) return ""
+    return message.parts
+        .filter((part) => part.type === "text")
+        .map((part) => (part as { text: string }).text)
+        .join("\n")
+}
+
+/** Only the user's typed text, without appended file content */
+export function getUserOriginalText(message: UIMessage): string {
+    return getMessageTextContent(message)
+        .replace(APPENDED_FILE_SECTIONS_PATTERN, "")
+        .trim()
+}