Move sidebar into layout files and fix timer cleanup across frontend

Sidebar and AdminSidebar were re-instantiated on every page navigation
(17 pages total), causing unnecessary DOM teardown/rebuild and redundant
localStorage reads. Now each lives in its respective +layout.svelte as a
single persistent instance.

Also adds onDestroy cleanup for leaked timers (settings, team, login RAF
loop) and CSS containment on <main> to isolate layout recalculations.

frontend/src/app.css

@@ -180,6 +180,11 @@ body {
 	50%       { transform: translateY(-6px); }
 }
 
+/* CSS containment — isolate layout/paint for independent UI regions */
+main {
+	contain: layout style;
+}
+
 /* Respect user motion preferences — covers both CSS class animations and inline style animations */
 @media (prefers-reduced-motion: reduce) {
 	*,