[Settings] Speed up page navigation; fix Advanced Paste double-load

[niels9001]

Summary

Settings pages — Advanced Paste in particular — felt slow on navigation and visibly "loaded twice". This PR addresses three coupled root causes.

What was happening

1. No frame caching. <Frame x:Name="shellFrame" /> had no caching and no page set NavigationCacheMode, so every nav click rebuilt a fresh Page instance, ViewModel, file watcher, and IPC subscriptions, and re-ran InitializeComponent on a 577-line / 41 KB XAML tree.
2. Cards entrance animation replayed on every nav. SettingsCardsAnimations bundled an EntranceThemeTransition (FromVerticalOffset=50) that fired every time a page came up.
3. Advanced Paste did duplicate dialog setup in Loaded. UpdatePasteAIUIVisibility() and UpdateFoundryLocalUIAsync() ran on every nav even though both only touch controls inside the hidden provider-configuration dialog. The Add/Edit handlers already call them right before ShowAsync(). The Loaded work produced a visible second reflow (the "double load") and, when the saved provider was Foundry Local, kicked off a process probe on every nav.

Changes

• NavigablePage.cs — Set NavigationCacheMode = NavigationCacheMode.Enabled in the base ctor. All pages inheriting NavigablePage (most settings pages) are now cached. Frame's default CacheSize=10 is enough to hold every top-level module. OnNavigatedTo and Loaded still fire on each visit, so existing per-nav logic (element-targeting animation, hotkey conflict refresh) keeps working.
• App.xaml — Removed EntranceThemeTransition from SettingsCardsAnimations. Kept RepositionThemeTransition so SettingsExpander still animates expand/collapse smoothly.
• AdvancedPastePage.xaml.cs —
  • Removed the duplicate UpdatePasteAIUIVisibility() + await UpdateFoundryLocalUIAsync() calls from the Loaded handler.
  • Added _isPasteAIProviderDialogOpen tracking, set when the dialog opens and cleared in its Closed handler.
  • RefreshEnabledState() now only runs the dialog-state updates while the dialog is actually open, so an IPC general-settings broadcast can no longer trigger a duplicate Foundry probe in the background.

Result

• Re-navigating between settings pages is essentially instant on the second visit.
• The visible "second load" reflow on Advanced Paste is gone.
• No more Foundry Local process probe on every Advanced Paste nav for users who picked it as their provider.
• Expander expand/collapse animations are unchanged.

Out of scope (potential follow-ups)

• Move heavy Advanced Paste ViewModel ctor work (InitializePasteAIProviderState, MigrateLegacyAIEnablement, PropertyChanged wiring) off the UI thread.
• Debounce the settings file watcher to suppress events that immediately follow our own writes.
• App-scope cache for Foundry Local availability so even the first dialog open is faster.
• Audit other heavy pages (FancyZones, MouseUtils, KeyboardManager, ZoomIt) for similar deferred-work opportunities — they all already benefit automatically from the new nav cache.

Validation

• Manual: nav between Dashboard ↔ Advanced Paste ↔ FancyZones repeatedly.
• IRefreshablePage path still triggers VM refresh on external general-settings updates.
• Note: a full local build was not completed in this environment due to a pre-existing C++ spdlog/fmt header incompatibility unrelated to these changes. The C# changes are isolated and were reviewed manually.