feat(ui): canvas-first redesign with a self-hosted draw.io

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
- the app's own canvas toolbar, shape library and page tabs replace
  draw.io's chrome
- new start screen, session rename and switcher, open .drawio files
- settings split into Models & keys, Appearance, Advanced and About
- phones switch between canvas and chat
- dark mode keeps the diagram on a white sheet and no longer flashes

With an external draw.io (NEXT_PUBLIC_DRAWIO_BASE_URL) the app falls back to
draw.io's own toolbar and 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/chat-helpers.ts

@@ -133,3 +133,37 @@ export function fixToolInputJson(input: string): string {
             .replace(/"(\w+)"\s*=\s*"/g, '"$1": "')
     )
 }
+
+// Shapes the user selected on the canvas; sent with the request as
+// selectedCells: [{ id, label }]
+const MAX_SELECTED_CELLS = 50
+const MAX_SELECTED_TEXT = 100
+
+function cleanSelectionText(value: unknown): string {
+    if (typeof value !== "string") return ""
+    return value
+        .replace(/[\r\n"]+/g, " ")
+        .trim()
+        .slice(0, MAX_SELECTED_TEXT)
+}
+
+/** Prompt text describing the selection, or "" when there is none */
+export function formatSelectionContext(selectedCells: unknown): string {
+    if (!Array.isArray(selectedCells)) return ""
+    const lines = selectedCells
+        .slice(0, MAX_SELECTED_CELLS)
+        .map((cell) => {
+            const id = cleanSelectionText(cell?.id)
+            if (!id) return null
+            const label = cleanSelectionText(cell?.label)
+            return label ? `- id="${id}" (${label})` : `- id="${id}"`
+        })
+        .filter((line): line is string => line !== null)
+    if (lines.length === 0) return ""
+    // Past the limit the model must know the list is cut short
+    const more = selectedCells.length - MAX_SELECTED_CELLS
+    if (more > 0) lines.push(`- and ${more} more selected shapes not listed`)
+    return `The user selected these shapes on the canvas before sending this message:
+${lines.join("\n")}
+Apply the request to the selected shapes unless it clearly asks for something else. Prefer edit_diagram with these cell ids over redrawing the whole diagram.`
+}