fix pixiv private timelines

[Tlaster]

Summary

• Add separate Pixiv private following and private bookmarks timelines using restrict=private.
• Add UiStrings entries and English titles for the new Pixiv private timeline entries across Android, Compose/Desktop, Web, and Apple.
• Preserve existing public Pixiv timeline ids and paging keys.

Validation

• ./gradlew :social:pixiv:compileKotlinJvm :compose-ui:compileKotlinJvm
• ./gradlew :app:compileDebugKotlin :desktopApp:compileKotlin
• env PATH=/Users/tlaster/.cache/codex-runtimes/codex-primary-runtime/dependencies/node/bin:/Users/tlaster/.cache/codex-runtimes/codex-primary-runtime/dependencies/bin:/usr/local/bin:/opt/homebrew/bin:/usr/bin:/bin:/usr/sbin:/sbin ./gradlew --no-daemon :web-shared:wasmJsBrowserDevelopmentLibraryDistribution
• ./node_modules/.bin/svelte-kit sync
• ./node_modules/.bin/svelte-check --tsconfig ./tsconfig.json
• jq empty appleApp/Shared/FlareAppleResource/Resources/Localizable.xcstrings
• xcodebuild -project appleApp/Flare.xcodeproj -scheme FlareAppleCore -configuration Debug -derivedDataPath /private/tmp/flare-apple-core build CODE_SIGNING_ALLOWED=NO

Fixes #2249

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Add Apple mappings for new Pixiv tab strings

When the Apple target imports this updated UiStrings enum, appleApp/Shared/FlareAppleCore/Sources/FlareAppleCore/PresentationMappings.swift:117-127 still exhaustively switches only the old cases and has no default. Adding these two cases without matching .pixivPrivateFollowing and .pixivPrivateBookmarks mappings breaks Apple builds that compile UiStrings.text, so the new Pixiv tabs need to be added to that Swift mapping as well.

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