feat(frontend): implement saved searches core logic and UI updates

[jcscottiii]

Title: feat(frontend): implement saved searches core logic and UI updates

Description:

This PR implements the core logic and UI updates for saved searches on the frontend, completing the migration to a unified search experience.

Why:
Previously, saved searches were tracked using a dedicated search_id URL parameter. This prevented users from combining a saved search with additional filters in the search bar. This PR migrates to using saved:ID and hotlist:ID directly within the q search parameter, enabling full composability of search terms.

Key Changes:

• Unified Search Query: Replaced search_id URL parameter with saved:ID (user searches) and hotlist:ID (global searches) within the q parameter. Updated app-bookmark-info-context.ts to parse these new tokens.
• API-Driven Global Searches: Removed hardcoded DEFAULT_GLOBAL_SAVED_SEARCHES constants. Global searches are now fetched dynamically from the backend via the new /v1/global-saved-searches endpoint.
• Removal of Client-Side Ordering: Deleted reorderByQueryTerms from webstatus-overview-data-loader.ts. Sorting by saved search order is now handled by the backend (implemented in PR feat(search): add query expansion for saved searches #2421), making client-side reordering redundant.
• UI Enhancements: Added loading skeletons for global saved searches in the sidebar to improve perceived performance.
• URL Upgrades: Added legacy support in OverviewPage to automatically detect search_id in the URL and redirect to the equivalent q=saved:ID format with a user toast notification.
• Test Coverage: Updated E2E and unit tests to reflect the new URL structure. Added waitForSidebarLoaded in E2E tests to reduce flakiness caused by visual regression checks firing before the sidebar hydrated.

Anti-Patterns / Known Debt:

• Dispatched a manual popstate event in urls.ts as a workaround to force Vaadin Router to update its location state. This is tracked in issue [ENHANCEMENT] Migrate off of deprecated router #2020 and should be removed when we migrate to a URLPattern-based router.