style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/src/components/PanelGroup.tsx

@@ -74,7 +74,9 @@ export default function PanelGroup({
             title={collapsed ? t("sidebar.expand") : t("sidebar.collapse")}
             className="text-muted hover:text-fg"
           >
-            <ChevronDown className={`w-4 h-4 transition-transform ${collapsed ? "-rotate-90" : ""}`} />
+            <ChevronDown
+              className={`w-4 h-4 transition-transform ${collapsed ? "-rotate-90" : ""}`}
+            />
           </button>
         )}
       </div>