refactor: implement icon rail sidebar for Design System v4

72px icon rail with hover flyouts, pin-to-expand toggle (260px),
keyboard accessible, mobile hamburger overlay. Flat TopBar styling
(no blur/glass). New BrandLogo mark (gradient square + lightning bolt).
BrandWordmark uses solid text color instead of gradient.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/index.css

@@ -261,6 +261,7 @@
     --sidebar-w: 260px;
   }
 
+  /* Legacy collapsed class — kept as alias for pinned inverse */
   .app-shell--collapsed {
     grid-template-columns: 56px 1fr;
   }