refactor(sidebar-filter): migrate to ReactiveElement with typed igc API

[viktorkombov]

Summary

• HTMLElement → ReactiveElement: lifecycle callbacks without a render cycle; createRenderRoot() returns this to keep children in light DOM.
• Proper igc types: replace ad-hoc cast types (ExpandableEl, FilterInputEl, etc.) with IgcInputComponent, IgcIconButtonComponent, IgcTreeItemComponent from igniteui-webcomponents.
• igc API usage: setExpandedState sets el.expanded directly on items instead of calling igcTree.expand(items) (no-op before the tree's slot registry is ready); ensureActiveAncestorsExpanded and collectAncestors use IgcTreeItemComponent.path; collectGroupDescendants uses getChildren({ flatten: true }).
• registerPageHooks() guard: global module-level document.addEventListener calls wrapped in a lazy _pageHooksRegistered guard — safe for multiple instances, clear intent.
• parseKeys/serializeKeys helpers: consolidate repeated split('\n').filter(Boolean) and [...keys].join('\n') patterns into two module-level functions.
• AbortController with { signal } for all event listener cleanup in disconnectedCallback.