fix(ui): remove default link underline from sidebar nav items

`.nav-item` was ported from a <button> but the new AppShell renders it as
an <a>, and the rule carried every button reset except `text-decoration`.
With no global anchor reset, the nav rows (Files, Shared, Recent, …) showed
the browser's default link underline. Add `text-decoration: none` to the
base rule (covers normal, hover and active states).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/styles/ported/sidebar.css

@@ -121,7 +121,9 @@
     color: var(--color-sidebar-text);
     font-size: 14.5px;
     font-weight: var(--weight-medium);
-    /* Button resets — .nav-item is a <button> (keyboard-operable nav). */
+    /* .nav-item is an <a> styled as a nav row — drop the default link underline
+       and reset the rest of the inherited link/button chrome. */
+    text-decoration: none;
     font-family: inherit;
     text-align: left;
     appearance: none;