Fix Xbox shell focus, search, and library filter navigation

[CyberoniOntoni]

Summary

Rebased onto current main (single commit, 29 files, +936/−76). Builds on merged #93 (controller shell), #97 (packaging), and #100 (CancellableLoad).

Adds a persistent shell search bar with suggestions and a full Search results page, fixes Xbox focus crashes and library filter controller navigation, and hardens item-details loading.

Follow-up: OSK dismiss (B) auto-selecting the first suggestion is fixed separately in #101 (merge after this PR).

Shell search

• AutoSuggestBox in MainPage with debounced suggestions (ShellSearchViewModel)
• Search page for full query results (SearchViewModel)
• SelectSuggestion consolidates suggestion pick (cancel in-flight fetch, clear field, navigate)
• NavigateToSearch reloads results when already on Search with the same query
• Search.OnNavigatingFrom cancels in-flight loads
• Async item resolution for suggestion navigation (NavigationManager.NavigateToItemAsync)
• CollectionNavigation routes collection folders to library views (single type→kind/title mapping)
• Nav menu clears selection while on Search (no stale highlight)

Xbox focus fixes

• Search is not auto-focused on page navigation; MainPage toggles IsTabStop instead of invalid FocusState.Unfocused (fixes Xbox 0xc000027b crash)
• Up from Home first row redirects focus to search via TrySetNewFocusedElement (SectionNavigationBehavior + TrapAtTop)
• Library filter XYFocusRight moved to code-behind (dynamic focus chain when filters are active)
• Search field cleared after navigating to an item

Load cancellation (#100 integration)

• ItemDetailsViewModel and SearchViewModel use CancellableLoad (not manual version counters)
• CancellableLoad.CancelAsync() added for ItemDetails.OnNavigatingFrom
• ItemDetailsViewModel resets display state on reload; null-safe media-source bindings

Test plan

• Search not auto-focused on page navigation; Up from Home focuses search
• Search suggestions navigate to item details without crash
• Re-submitting the same search query reloads results
• Nav menu has no stale highlight while on Search
• Opening Movies from Home no longer crashes (0xc000027b)
• Library filter toolbar D-pad navigation works with/without active filters
• Release x64 build passes (0 warnings, 0 errors)

[CyberoniOntoni]

There's going to be a follow-up PR after #101 (which fixes search panel behavior), with a bugfix for a seek bar (control bar) not autohiding during playback.