Fix: dialog scroll, isDirty false positive, mobile edit btn, widget copy

Five fixes:

1. Dialog mobile scroll: DialogContent now has max-h-[calc(100dvh-2rem)]
   overflow-y-auto so dialogs that don't fit on screen can scroll
   instead of clipping their footer (and Cancel button) off-screen.

2. isDirty false positive: WidgetConfigDialog's SheetForm used
   isDirty={draft !== null} which was true the moment you opened edit
   mode, even with no changes. Now stores a draftBaseline at startEdit
   time and compares JSON.stringify(draft) !== JSON.stringify(baseline).
   The discard-confirmation only appears when something actually changed.

3. Mobile edit button always visible: the widget card's edit button was
   opacity-0 group-hover:opacity-100 (hover-only). Changed to
   md:opacity-0 md:group-hover:opacity-100 — always visible below md,
   hover-reveal at md+.

4. Copy button for referenced widgets: WidgetInstanceCard gains an onCopy
   prop. On the Dashboard, referenced widgets get a Copy icon button that
   triggers detachRef (creates an independent clone). The edit button on
   referenced widgets edits the original (shared config).

5. ConfirmDialog Cancel: fixed by #1 (the Cancel button was off-screen
   on mobile dialogs that couldn't scroll).

128 tests pass; lint/build green.

frontend/src/components/ui/dialog.tsx

@@ -59,7 +59,7 @@ function DialogContent({
       <DialogPrimitive.Content
         data-slot="dialog-content"
         className={cn(
-          "fixed top-1/2 left-1/2 z-50 grid w-full max-w-[calc(100%-2rem)] -translate-x-1/2 -translate-y-1/2 gap-4 rounded-xl bg-popover p-4 text-sm text-popover-foreground ring-1 ring-foreground/10 duration-100 outline-none sm:max-w-sm data-open:animate-in data-open:fade-in-0 data-open:zoom-in-95 data-closed:animate-out data-closed:fade-out-0 data-closed:zoom-out-95",
+          "fixed top-1/2 left-1/2 z-50 grid w-full max-w-[calc(100%-2rem)] max-h-[calc(100dvh-2rem)] overflow-y-auto -translate-x-1/2 -translate-y-1/2 gap-4 rounded-xl bg-popover p-4 text-sm text-popover-foreground ring-1 ring-foreground/10 duration-100 outline-none sm:max-w-sm data-open:animate-in data-open:fade-in-0 data-open:zoom-in-95 data-closed:animate-out data-closed:fade-out-0 data-closed:zoom-out-95",
           className
         )}
         {...props}