refactor(chat): drop redesign leftovers and old-format migrations

- stores/versions-store.ts, hooks/use-session-manager.ts,
  lib/session-storage.ts: remove the diagramHistory conversion layer,
  the localStorage chat migration and the empty templates store
- stores/canvas-store.ts, lib/drawio/editor-bridge.ts: remove the
  pages list nothing read
- components/chat/chat-engine.tsx: use the context's chartXMLRef and
  latestSvgRef instead of local copies, subscribe to one settings field,
  stop prefetching quota limits (lib/use-quota-manager.tsx takes the
  values from the 429 body), readDiagramFile delegates to the MCP
  loader (which now strips XML prologs and accepts <svg:svg>)
- contexts/diagram-context.tsx: one requestTaggedExport path for
  thumbnails, validation, fetch and save; fetchDiagramXml replaces
  handleExport plus the exported resolver map
- hooks/use-validate-diagram.ts and app/api/validate-diagram: plain
  fetch returning JSON instead of a streamed object nobody consumed
- hooks/use-diagram-tool-handlers.ts: drop the DEBUG blocks; canvas
  vars inheritance lives in placeOnPage (lib/diagram-pages.ts)
- lobby-hero, icon-button, drawio-frame: remove unreachable branches
  and unused props

components/workspace/icon-button.tsx

@@ -13,9 +13,7 @@ interface IconButtonProps extends React.ComponentProps<"button"> {
     label: string
     /** Shown next to the label in the tooltip, e.g. "⌘B" */
     shortcut?: string
-    active?: boolean
     tooltipSide?: "top" | "bottom" | "left" | "right"
-    size?: "sm" | "md"
 }
 
 /** Square icon button with a tooltip; the tooltip text is also its label */
@@ -24,9 +22,7 @@ export const IconButton = forwardRef<HTMLButtonElement, IconButtonProps>(
         {
             label,
             shortcut,
-            active = false,
             tooltipSide = "bottom",
-            size = "md",
             className,
             children,
             ...props
@@ -40,17 +36,12 @@ export const IconButton = forwardRef<HTMLButtonElement, IconButtonProps>(
                         ref={ref}
                         type="button"
                         aria-label={label}
-                        aria-pressed={active || undefined}
                         className={cn(
-                            "inline-flex shrink-0 items-center justify-center rounded-lg text-muted-foreground transition-colors",
+                            "inline-flex size-8 shrink-0 items-center justify-center rounded-lg text-muted-foreground transition-colors",
                             "hover:bg-accent hover:text-foreground",
                             "focus-visible:outline-2 focus-visible:outline-offset-1 focus-visible:outline-ring",
                             "disabled:pointer-events-none disabled:opacity-35",
                             "[&_svg]:size-4 [&_svg]:stroke-[1.75]",
-                            size === "md" ? "size-8" : "size-7",
-                            // A tinted tile: solid ink is for the main action
-                            active &&
-                                "bg-interactive-active text-foreground hover:bg-interactive-active [&_svg]:stroke-2",
                             className,
                         )}
                         {...props}
@@ -73,8 +64,3 @@ export const IconButton = forwardRef<HTMLButtonElement, IconButtonProps>(
         )
     },
 )
-
-/** Thin vertical divider between toolbar groups */
-export function ToolbarDivider() {
-    return <span className="mx-1 h-5 w-px shrink-0 bg-border" aria-hidden />
-}