feat: dimensions folder grid UI overhaul

- Folder cards: off-black bg (--rah-bg-folder), compact 80px height
- Icon + name in footer, green pill badge for node count
- Description snippet shown below name when set
- Hover-only edit/delete actions
- Add --rah-bg-folder CSS variable to both themes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/globals.css

@@ -47,6 +47,7 @@ html[data-theme="dark"] {
   --rah-bg-elevated: #161616;
   --rah-bg-active: #1a1a1a;
   --rah-bg-modal: #141414;
+  --rah-bg-folder: #222222;
   --rah-border: #1a1a1a;
   --rah-border-strong: #2a2a2a;
   --rah-border-stronger: #3a3a3a;
@@ -77,6 +78,7 @@ html[data-theme="light"] {
   --rah-bg-elevated: #ececec;
   --rah-bg-active: #e8e8e8;
   --rah-bg-modal: #ffffff;
+  --rah-bg-folder: #dfdfdf;
   --rah-border: #e5e5e5;
   --rah-border-strong: #d4d4d4;
   --rah-border-stronger: #bdbdbd;