polish(ui): founder feedback batch - inset sidebar hairline, calmer dialogs, Discord mark, chat skeleton (#1158)

- Sidebar: the hairline above the user block is inset to the content
  edges instead of running edge-to-edge (concept language).
- User menu: the Discord community link renders the actual Discord mark
  (inlined simple-icons path, CC0) instead of lucide MessagesSquare.
- Dialogs: open/close animation toned down, zoom 98 instead of 95 and
  150ms instead of 200ms.
- Settings modal: switching tabs no longer remounts the intercepted
  route (and replayed the whole open animation on every click). The rail
  now swaps sections via history.replaceState, which Next syncs into
  usePathname(), so the dialog stays mounted and tab switches are
  instant. Verified via Playwright: dialog DOM node survives three tab
  switches, URL tracks the section, Esc still closes back.
- /chat loading: the shared dashboard skeleton stretched edge-to-edge in
  chat's full-bleed wrapper (chat's own layout is what suspends, so a
  chat/loading.tsx cannot catch it). The shared fallback is now route-
  aware and renders a two-pane chat silhouette for /chat.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/ui/dialog.tsx

@@ -37,7 +37,7 @@ const DialogContent = React.forwardRef<
     <DialogPrimitive.Content
       ref={ref}
       className={cn(
-        "fixed left-[50%] top-[50%] z-50 grid w-full max-w-lg translate-x-[-50%] translate-y-[-50%] gap-4 border bg-background p-6 shadow-[var(--shadow-md)] duration-200 max-h-[calc(100dvh-2rem)] overflow-y-auto scrollbar-visible data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 sm:rounded-lg",
+        "fixed left-[50%] top-[50%] z-50 grid w-full max-w-lg translate-x-[-50%] translate-y-[-50%] gap-4 border bg-background p-6 shadow-[var(--shadow-md)] max-h-[calc(100dvh-2rem)] overflow-y-auto scrollbar-visible data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-98 data-[state=open]:zoom-in-98 sm:rounded-lg",
         className
       )}
       {...props}