Add library search and metadata filtering

[davidhoang]

Summary

• Add toolbar search that matches filenames, comments, where-from text, tags, and labels
• Add a filter menu for file type and locally stored tags, with a clear-filters action
• Show a dedicated "No matches" empty state when filters exclude all images
• Improve offline/iCloud empty states with Try Again and iCloud-specific copy
• Add unit tests for LibraryFiltering and UI tests for search/filter controls

This branch also includes the navigation polish from #12 (native sidebar toggle, NavigationStack detail push, inspector persistence). Merge #12 first if you prefer smaller review chunks, or land both together.

Test plan

• Search by filename, comment, tag, and label in a populated library
• Filter by file type and tag via the toolbar filter menu
• Clear filters and confirm the full library returns
• Confirm "No matches" appears when filters exclude everything
• Try Again on an offline external drive folder
• Run UI tests (testPrimaryNavigationControls, testInspectorCanBeToggled)
• Run unit tests (testLibraryFilteringMatchesNamesTypesAndMetadata)

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