feat: offline search — SwiftUI feature layer

[danieltmbr]

Summary

Stacks on #228 (persistence & networking layer).

• Per-tab .searchable bars on Blog, Catalogue, and Quotes — each filters only its own content, respecting active language/category/source filters
• Dedicated Search tab with sectioned results (Posts / Catalogue / Quotes) and a scope bar; catalogue section surfaces note-body snippets
• CatalogueSearchEngine: @MainActor @Observable engine that drives catalogue results via CatalogueStore.search; needed because @Query can't follow the PreviewRecord→NoteRecord UUID link
• BlogSearchAction / CatalogueSearchAction / QuotesSearchAction: environment-injected seams — local predicate query runs first for instant cached results, then the network search upserts new data and triggers a second local pass
• Detail section fix: all six typed sections (Song/Album/Book/Movie/Podcast/Playlist) now accept item: PreviewRecord and coalesce typed + preview fields so a half-materialised item (preview exists, typed record not yet cached) renders a header immediately; .task fires a single-item fetch on appear when the typed record is absent

Test plan

• Xcode build passes for app-core and Reader target
• Blog tab: search term filters posts; results update after network search upserts previously-unsynced posts
• Catalogue tab: title match, subtitle match, and note-body match all return results; category + language filters narrow while searching
• Quotes tab: body search works; source + language filters apply during search
• Search tab: sectioned results render; scope bar narrows correctly; catalogue section shows note snippets
• Airplane mode: repeat a previously-run search — cached results (including note snippets) render without network
• Half-materialised catalogue item: search a term matching only a note body of an unsynced item, tap the result — preview header renders immediately, enriches once the typed-record fetch completes

Note: If the app crashes on launch with a SwiftData migration error due to the secondaryInfo optionality change (from #228), delete the app to reset the cache — Reader's DB is a disposable network cache.

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