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/components/message/layout/layout.css.ts

@@ -204,6 +204,14 @@ export const Username = style({
     'button&:hover, button&:focus-visible': {
       textDecoration: 'underline',
     },
+    // Stationery uses a colored border-bottom accent instead
+    'html.stationery &, body.stationery &': {
+      overflow: 'visible',
+      overflowY: 'visible',
+    },
+    'body.stationery button&:hover, body.stationery button&:focus-visible': {
+      textDecoration: 'none',
+    },
   },
 });