Add Stationery and Stationery Dark themes with notebook chrome.

Manila folders, ruled chat, post-it nav, and folder tabs; dark variant uses Catppuccin Mocha colors with muted stickies and soft glows.

src/app/features/room/RoomInputPlaceholder.css.ts

@@ -5,6 +5,6 @@ export const RoomInputPlaceholder = style({
   minHeight: toRem(48),
   backgroundColor: color.SurfaceVariant.Container,
   color: color.SurfaceVariant.OnContainer,
-  boxShadow: `inset 0 0 0 ${config.borderWidth.B300} ${color.SurfaceVariant.ContainerLine}`,
-  borderRadius: config.radii.R400,
+  borderTop: `${config.borderWidth.B300} solid ${color.SurfaceVariant.ContainerLine}`,
+  borderRadius: 0,
 });